Endoscopic Ultrasound (EUS) is a medical device that integrates ultrasound and endoscopy, placing a miniature high-frequency ultrasound probe at the front of the endoscope, which allows both direct observation of the luminal morphology and real-time ultrasound scanning to obtain ultrasound images of the histological features of the canal wall at all levels and the surrounding adjacent organs. EUS is an organic combination of endoscopic technology and ultrasound technology, which opens up new avenues for the diagnosis and treatment of GI tract diseases. Usually, the digestive tract can be divided into five layers, from the inner to the outer layers: the mucosal layer, the myxomucosal layer, the submucosal layer, the intrinsic muscle layer, and the plasma membrane layer. Although gastroscopy and colonoscopy can make direct judgments on most gastrointestinal diseases, the naked eye can only see the most superficial layer of the digestive tract (mucosal layer), and for those lesions that originate from the submucosa (including the myxomucosa, submucosa, intrinsic muscle layer, and plasma layer), the surface is smooth and ordinary gastroscopy cannot make a diagnosis; there are also some lesions such as the retroperitoneum, especially those of the pancreas, which are located outside the stomach cavity adjacent to the stomach wall, which is even more difficult to diagnose with ordinary gastroscopy. The specific uses of ultrasound endoscopy: 1, pancreatic lesions: ultrasound endoscopy was originally conceived for pancreatic lesions, due to the deep location of the pancreas, often found at a late stage, the survival period is short, so in order to early detection of pancreatic lesions, there is ultrasound endoscopy, with the continuous progress of medical technology and progress, the current ultrasound endoscopy can identify the benign and malignant pancreatic tumors through puncture biopsy, can find 1 cm or even In addition, ultrasound endoscopy can be used to identify small pancreatic lesions below 1 cm or even 1 cm, to evaluate the possibility of surgical resection and prognosis, and to help choose the treatment plan. In addition, the diagnosis of chronic pancreatitis is a better auxiliary diagnosis and differential diagnosis method, which can better reflect the changes in the pancreatic parenchyma. 2.Esophageal, gastric and intestinal augmentation lesions: especially for submucosal tumors (such as lipoma, mesenchymal tumor, etc.); ultrasound can determine whether the lesion is in the wall of the digestive tract itself or outside the wall and whether it is compressed by extra-mural organs; if the lesion is in the wall of the digestive tract, the exact origin, nature, scope and size of the lesion can be determined, which can help in the selection of treatment plan. 3.Malignant tumors of GI tract (such as esophageal cancer, gastric cancer, colon and rectal cancer): the staging of tumors can be performed, especially the T-staging of early esophageal cancer, gastric cancer and colorectal cancer, in order to assess whether minimally invasive endoscopic treatment is feasible, and if the tumor is in the progressive stage, the resectability of surgery, prognosis and the choice of treatment plan can be assessed. 4.Biliary system diseases: the diagnosis of common bile duct stones is highly sensitive, specific and non-invasive, and can determine the site and size of biliary system tumors; perform staging, assess resectability, prognosis and guide treatment. 5.Posterior mediastinal lesions can be detected and fine-needle aspiration biopsy can be performed to clarify the diagnosis.