With the development of economy, improvement of living standard and change of lifestyle as well as the aggravation of environmental pollution, the incidence of tumor is also increasing. Many malignant tumors, especially gastrointestinal tumors, are already in advanced stage once clinical symptoms appear. Early detection, early diagnosis and early treatment of gastrointestinal tumors is an important element in tumor prevention and treatment. Among the gastrointestinal tumors, gastric cancer is one of the most common tumors. The importance of early diagnosis of gastric cancer The incidence of gastric cancer is the 4th most common cancer in the world, and the 2nd most common cause of cancer-related death, with about 700,000 deaths due to gastric cancer each year. The overall 5-year survival rate of gastric cancer in Japan is 40-60%, which is the highest in the world, and about 20% in other countries. In China, about 227,000 people die of gastric cancer every year, accounting for 23% of all malignant tumor deaths, while the early diagnosis rate of patients is less than 1/10, and the surgery rate of early gastric cancer (EGC) is less than 5-10%. In terms of prognosis, EGC has a good prognosis with a 5-year survival rate of over 90%, while the 5-year survival rate of progressive gastric cancer is only 30-40%. Therefore, early diagnosis and early treatment of gastric cancer are of great importance to improve the efficacy and reduce the mortality rate. Techniques for early diagnosis of gastrointestinal tumors 1. Screening for high-risk groups of gastric cancer High-risk groups mainly refer to patients with pre-cancerous diseases and pre-cancerous lesions. Precancerous diseases refer to benign gastric diseases related to gastric cancer, including chronic atrophic gastritis, gastric polyps, residual gastritis and gastric ulcer. Pre-cancerous disease refers to the pathological histological changes that can easily turn into cancerous tissue, including the following sequential process: normal → proliferation → low-grade intraepithelial neoplasia → high-grade intraepithelial neoplasia → infiltrating cancer. For some high-risk groups and precancerous lesions, they should be followed up regularly for early intervention and treatment. 2.Gastroscopy (1) General electron gastroscopy EG endoscopy is insidious and has no obvious features, mainly roughness of mucosa, easy bleeding when touched, patchy congestion and mucosal erosion. Typical gastric cancer can be seen as nodules, masses, uneven cancerous ulcers, etc. （EUS can also be used to determine whether there are regional lymph node metastases. EUS can also be used to accurately stage gastric cancer to guide treatment and for postoperative follow-up to detect residual or recurrent cancer. Due to the limitation of ultrasound beam penetration distance, the large part of the right lobe of the liver, the retroperitoneum and mesenteric lymph nodes below the superior mesenteric vessels in the abdominal cavity cannot be detected by EUS, so EUS cannot provide conclusive diagnosis of distant metastases. In recent years, with the continuous development of science and technology, some new gastroscopic diagnostic techniques have gradually emerged and are being gradually promoted and tried in clinical practice to accumulate experience and knowledge. These new techniques are: (3) Pigmented endoscopy Pigmented endoscopy refers to the use of pigmented preparations and dyes on the basis of routine endoscopic examination to increase the contrast between lesions and normal tissues, so as to make the morphology and scope of lesions clearer, thus improving the visual detection of gastric cancer, guiding biopsy and treatment, and increasing the detection rate of lesions. (4) Magnification endoscopy Magnification endoscopy can magnify the endoscopic image tens to hundreds of times, which can clearly show the microstructural changes such as the opening of glandular ducts and microvasculature in the mucosa of the gastrointestinal tract. (5) Fluorescence endoscopy The principle is that compounds in biological tissues react with specific wavelengths of luminescent substances and can emit special fluorescent signals. The biochemical characteristics of benign and malignant lesions are different, and the corresponding fluorescence spectra have specificity. Fluorescence endoscopy can clearly show early tumors of the gastrointestinal tract and the degree of mucosal infiltration, but is not as specific for superficial gastric tumors. Autofluorescence endoscopy has a strong advantage in guiding biopsies. (6) Infrared electronic endoscopy The advantage is that infrared light can penetrate tissues. Using electronic endoscopy with far-infrared light source of 780-840 nm, the morphology of fine blood vessels of mucosa can be seen, which can be used to distinguish mucosal carcinoma, submucosal carcinoma and progressive carcinoma; it can also clearly show the condition of blood vessels under the gastric mucosa. 3.Imaging (1) Barium meal imaging Conventional barium meal is not enough to show the microstructure of gastric mucosa, nowadays, double contrast barium meal examination is mainly used to detect various types of EGC, including small gastric cancer and micro gastric cancer with maximum diameter < 1 cm or even 0.5 cm. Endoscopy is required when there are suspicious findings. (2) CT, especially spiral CT, has certain value in the diagnosis of gastrointestinal tumor, which can better observe the situation inside and outside the gastrointestinal tract and whether there is metastasis in the distant organs. CT simulates endoscopy and can accurately measure the size of the lesion, and the combination with gastroscopic biopsy can improve the correct diagnosis rate of EGC. 4.Pathological diagnosis This examination is the gold standard for the diagnosis of gastric cancer, and multiple sampling during endoscopy can improve the accuracy of diagnosis. The depth of infiltration and the relationship with the surrounding area are affected by the depth of sampling and need to be combined with other means for comprehensive analysis.