Carcinoid tumors can occur in any part of the digestive tract except the esophagus, 1/3 of them are located in the appendix, mostly in females, and rarely metastasize. The rest are located in the duodenum, colon (mostly in men), gallbladder and Merkel’s diverticulum, etc. They can also occur in the bronchi and ovaries, but are less common. In the small intestine, the malignancy is large and can metastasize to the lungs, bones and other intra-abdominal organs. Bronchial carcinoid tumors with carcinoid syndrome have a poor prognosis. Carcinoid tumors can occur at any age. Carcinoid tumors of the appendix occur at the age of 30 to 40 years old, while carcinoid tumors of other sites occur at the age of 50 to 60 years old. Carcinoid tumor cells can produce many biologically active substances, the most important of which are 5-hydroxytryptamine, bradykinin, histamine and prostaglandin, etc. 5-hydroxytryptamine is derived from tryptophan in food. After the development of carcinoid tumors, 60% of the tryptophan in food is converted into 5-hydroxytryptamine in carcinoid tumor cells, so there is a significant increase of 5-hydroxytryptamine in the blood of carcinoid tumor patients. Bradykinin is another relatively major bioactive substance causing clinical manifestations. In carcinoid tissues, there is a large amount of vasodilator, a protein hydrolase that acts on kininogen to produce lysergic acid bradykinin (tryptokinin), which is converted to bradykinin by the action of aminopeptidase. Bradykinin causes skin flushing and skin temperature is not high, which is the cold type. 5-hydroxytryptophan causes skin flushing and skin warmth, which is the hot type. If liver metastasis is already present, on the one hand, because of the excessive production of 5-hydroxytryptamine, on the other hand, it can directly enter the hepatic vein and enter the body circulation. Furthermore, the liver has a reduced clearance function due to the extensive carcinoid invasion. Therefore, carcinoid syndrome can occur after the presence of liver metastases.