It is well known that gastroscopy and colonoscopy can enter the body through the mouth and anus, respectively, to search for gastrointestinal lesions. Ultrasonography generally involves placing the probe outside the body and probing the abdominal organs. Sometimes the abdominal organs are far from the abdominal wall or there is a lot of gas in the intestinal canal, which can affect the observation of the organs by ultrasound. Gastroscopy and enteroscopy can only see the most superficial layer of the digestive tract (mucosal layer), some lesions have smooth surfaces and originate from the submucosa (e.g. submucosa, muscular layer, plasma layer); or some lesions are located outside the gastric cavity close to the gastric wall, when combining the functions of endoscopy and ultrasound together, the lesions can be better observed and judged, and further treated. Endoscopic ultrasonography (EUS) examination is a combination of ultrasound probe and endoscopic technology, and when the endoscope is delivered into the digestive tract, both direct observation of mucosal surface lesions through the endoscope and real-time ultrasound scanning can be performed to observe the ultrasound images of the tissue structure of each layer of the digestive canal wall and its adjacent organs, further improving the diagnostic endoscopy and ultrasound The level of endoscopy and ultrasound is further improved. Ultrasound endoscopy is currently the most promising new technology in gastrointestinal endoscopy. With the development of ultrasound and endoscopy technology, more and more diseases of the digestive tract and adjacent organs can be detected by ultrasound endoscopy, thus the scope of indications for ultrasound endoscopy has been expanded. (1) Malignant tumors of the GI tract (e.g., esophageal, gastric, colon, rectum): TNM staging is performed, but it is of limited use for M-staging to assess surgical resectability, prognosis, and to guide the selection of treatment options. (2) Submucosal tumors (e.g., smooth muscle tumors, etc.): to determine whether the lesion is extramural, organ compression, or the duct wall itself; to determine the exact origin, nature, and extent of the lesion; and to guide the choice of treatment plan. (3) Pancreatic lesions: It is a better auxiliary diagnosis and differential diagnosis method for the diagnosis of chronic pancreatitis, and can better reflect the changes of pancreatic parenchymal structure. It can identify benign and malignant pancreatic tumors, detect tumors of 1 cm or even below, and determine the type of tumor through ultrasound-guided puncture biopsy; assess the possibility of surgical resection, prognosis, and help choose the treatment plan. (4) Biliary system diseases: Diagnosis of common bile duct stones is as sensitive and specific as ERCP, but non-invasive. It is sensitive to the diagnosis of biliary tract tumors and can determine the site and size of the tumor; perform TNM staging, assess resectability, prognosis and guide treatment. (5) Ulcer disease: determine ulcer staging, guide treatment, and judge the quality of ulcer healing. (6) To determine the degree of esophageal varices and the efficacy of embolization therapy. (7) It can show some mediastinal lesions. 2.Contraindications The contraindications of ultrasound endoscopy of the digestive system are basically the same as those of general gastroscopy. (1) Absolute contraindications include: ① Severe cardiopulmonary disorders that cannot tolerate endoscopy. (2) Those who are in critical condition such as shock. (3) Those who are suspected of having gastric perforation. ④ Uncooperative psychiatric patients or those with severe mental retardation. ⑤ Acute inflammation of the oral cavity, pharynx, esophagus and stomach, especially corrosive inflammation. (6) Others: obvious thoracic aortic aneurysm, cerebral hemorrhage, etc. (2) Relative contraindications are: ① Giant esophageal diverticulum, obvious esophageal varices or high-grade esophageal cancer, and high spinal deformity. (2) Those with cardiac and other important organ insufficiency. ③ Those who have uncontrolled hypertension. II. Diagnostic value 1. Submucosal tumor: EUS is the preferred method to diagnose submucosal tumor. Through EUS, we can easily exclude the artifacts caused by extraluminal compression. For submucosal tumor, it can clearly show the lesion originated from the layer of the digestive tract wall, the size, shape, margin and echogenicity of the lesion. 2.Esophageal cancer: the most important feature of EUS is to determine the depth of lesion infiltration into the wall and whether there are enlarged lymph nodes around; EUS can show the depth of invasion of esophageal cancer (T) more accurately; EUS is much better than CT for the diagnosis of tumor lymph node metastasis (N), and EUS can detect lymph nodes of 2-3mm in size. Currently, EUS-guided fine-needle aspiration cytology of lymph nodes is the best method to determine the benignity and malignancy of lymph nodes before surgery. In conclusion, in the TNM staging of esophageal cancer, EUS is obviously due to CT and MRI for the determination of T and N, but EUS is inferior to CT and MRI for the determination of distant tumor metastasis (M). 3.Gastric cancer: the staging scheme of gastric cancer and esophageal cancer is similar. the overall accuracy of EUS in determining the depth of invasion (T) of gastric cancer reaches 84%, the sensitivity of diagnosing lymph node metastasis (N) is 81%, and the specificity is 50%. for the determination of distant metastasis (M) of tumor, EUS is inferior to CT and magnetic resonance imaging. For gastric cancer cases confirmed by gastroscopy and biopsy, the main value of EUS is the TNM stage of gastric cancer, while for patients with invasive gastric cancer (leathery stomach), especially those with negative endoscopic biopsy results, EUS is the preferred examination method. 4.Pancreatic diseases: Traditional pancreatic examination methods include body ultrasound, CT, MRI and retrograde pancreatic ductography (ERP). The application of pancreatic EUS must be contrasted with the traditional pancreatic examination methods to complement each other in order to arrive at the most optimal diagnosis. EUS is now recognized as the most sensitive method for detecting microscopic pancreatic lesions, which can be detected in as little as 2-3 mm. Fine needle aspiration cytology under the guidance of EUS can make a qualitative diagnosis of early microscopic lesions in the pancreas. The application of EUS can perform TNM staging of pancreatic cancer and determine the possibility of surgical resection. For pancreatic cysts, EUS not only has diagnostic value, but also can be used for treatment such as internal drainage by puncture. 5.Mediastinal lesions: The diagnosis of mediastinal lesions currently mainly relies on non-invasive examinations such as CT and MRI, but EUS has an important role in the localization and qualitative diagnosis of mediastinal lesions, especially for the lesions in the middle and posterior mediastinum around the esophagus, which are more accurate. The most clinically valuable is fine-needle aspiration cytology under EUS guidance, which is important for the diagnosis of many diseases such as metastatic lymph nodes, lymphoma, and nodal disease in malignant tumors of unknown origin in the mediastinum. In addition, EUS has an important role in the evaluation of portal hypertension and the quality of ulcer healing. For example, gastroscopy revealed a bulging lesion in the esophagus but with a smooth surface. Ultrasound endoscopy revealed that the lesion originated from the mucosal muscle layer and was convex to the lumen of the esophagus, and smooth muscle tumor was considered suitable for endoscopic resection and treatment, so endoscopic treatment was performed, and the postoperative wound was intact and safe.