Neuroendocrine tumors of the gastrointestinal tract include neuroendocrine tumors that occur in the stomach, small intestine, rectum, and appendix. According to the latest WHO 2010 edition classification, gastrointestinal neuroendocrine tumors are classified into highly differentiated neuroendocrine tumors (including G1 and G2) and poorly differentiated neuroendocrine carcinomas (G3). Highly differentiated neuroendocrine tumors of the gastrointestinal tract (G1), also called carcinoid tumors, are less malignant. Patients with carcinoid tumors present with endocrine hormone-related symptoms called carcinoid syndrome. Typical carcinoid syndrome includes episodes of skin flushing, diarrhea, abdominal pain, wheezing, and heart valve lesions. Gastrointestinal neuroendocrine tumors are relatively rare and the commonly used blood tumor markers CEA, CA125 and CA199 are often not high. The blood marker for neuroendocrine tumors called CgA (chromogranin A) can be used for screening, diagnosis and disease monitoring of gastroenteropancreatic neuroendocrine tumors. Patients with gastric neuroendocrine tumors should also be tested for serum gastrin, which is meaningful for both staging and diagnosis and disease monitoring. (5-hydroxyindoleacetic acid) is a metabolite of 5-hydroxytryptamine and is excreted in the urine. In neuroendocrine tumors with carcinoid syndrome, urinary 5-HIAA levels are significantly elevated. Measurement of 24-hour urinary 5-HIAA can be used to monitor the condition of carcinoid syndrome and assess the efficacy of treatment. Therefore, when gastrointestinal neuroendocrine tumors are reviewed, serum CgA and urinary 5-HIAA should be tested in addition to blood CEA. patients with gastric neuroendocrine tumors should be tested for serum gastrin before treatment so that they can be typed (type 1, type 2 and type 3). Those with normal gastrin levels belong to type 3, which is more malignant and has a different treatment strategy than type 1 and type 2.