There is a type of master cell in the gastric mucosa whose main function is to secrete proteases. The human gastric gland contains a kind of urinary acid gland, the gland contains three kinds of cells, mainly divided into the main cells, wall cells, mucus cells. Among them, the master cell mainly secretes pepsin, which facilitates the digestion of proteins in food and can break down proteins into polypeptides, which facilitates intestinal absorption in the human body. After being secreted, pepsinogen needs to be activated into pepsin under the action of gastric acid. Pepsin is most active when the pH of gastric acid is between 1.8 and 3.5, and less active when the pH of gastric acid is higher. If the decrease of pepsinogen is found during the examination, it means that the gastric mucosa is atrophied, and the patient should be further examined in order to avoid gastric cancer.