Gastroscopy is the simplest examination method for gastric cancer. It is very important in diagnosing gastric cancer because it can directly observe whether there is tumor on the gastric mucosa, and biopsies can be extracted from the tumor for pathological examination, once cancer cells are found in the tissues, it indicates that gastric cancer is already present. Gastroscopy can not only diagnose gastric cancer, but also understand the extent of gastric cancer invasion to the stomach and stage gastric cancer. Most gastric cancers develop gradually from atrophic gastritis, intestinalization and abnormal hyperplasia, and this process has a long period of time. In addition to gastroscopy, serum gastrin test and upper gastrointestinal barium meal test can also be used to detect gastric cancer. These two tests can understand the degree of atrophy of gastric mucosa and make preliminary judgment on early gastric cancer.