Apart from esophagus, carcinoid tumors can occur in any part of the digestive tract, with the appendix accounting for 1/3 of the cases, most common in females, and seldom metastasized. Those located in the stomach account for 2%, and the rest are located in the duodenum, colon (most common in males), gallbladder and Meckel’s diverticulum, etc. They can also occur in the bronchus and ovary, but it is less common. Those that occur in the small intestine are highly malignant and may metastasize to the lungs, bones, and other intra-abdominal organs. Bronchial carcinoid tumors associated with carcinoid syndrome have a poor prognosis. Carcinoid tumors can occur at any age. Carcinoid tumors in the appendix usually occur at the age of 30-40 years, while carcinoid tumors in other parts of the body usually occur at the age of 50-60 years. Carcinoid tumor cells can produce many kinds of biologically active substances, the most important of which are 5-hydroxytryptamine, bradykinin, histamine and prostaglandins, etc. 5-hydroxytryptamine comes from tryptophan in food. After the occurrence of carcinoid tumor, 60% of food tryptophan is converted to 5-hydroxytryptophan in carcinoid tumor cells, therefore, 5-hydroxytryptophan in the blood of carcinoid tumor patients has obvious increase. Diagnosis can be made on the basis of episodic skin flushing, diarrhea and increased 5-hydroxyindoleacetic acid in urine. Bradykinin is another relatively major biologically active substance that causes clinical manifestations. In carcinoid tissues, there is a large amount of vasopressin, a protein hydrolyzing enzyme, which acts on the kininogen and produces lysyl bradykinin (tryptic kinin), which is converted to bradykinin by the action of aminopeptidase. Bradykinin causes flushing of the skin and low skin temperature, which is the cold type, while 5-hydroxytryptamine causes flushing of the skin and warming of the skin, which is the hot type. If liver metastases are already present, this is due to the excessive production of 5-hydroxytryptophan on the one hand, and on the other hand, it can go directly into the hepatic veins and enter the body circulation. Furthermore, the liver has a reduced clearance function due to extensive carcinoid invasion. Therefore, carcinoid syndrome can occur after the presence of liver metastases.