Carcinoembryonic antigen (CEA) is generally regarded as a tumor marker in the digestive tract, but its increase does not necessarily indicate a tumor, such as some benign inflammatory diseases, smoking and drinking can cause a mild increase. If CEA is found to be increased in physical examination, on the one hand, regular examination of stool routine, colonoscopy, gastroscopy, chest X-ray, etc., on the other hand, pay attention to diet and some bad habits. The diagnosis of tumor is a comprehensive consideration. CEA is a polysaccharide protein complex with a molecular weight of 22ku, 45% of which is protein. the gene encoding CEA is located on chromosome 19. Generally, CEA is synthesized by cells of fetal gastrointestinal epithelial tissue, pancreas and liver. CEA is a non-organ-specific tumor-associated antigen, and most of the tumors secreting CEA are located in cavernous organs, such as gastrointestinal tract, respiratory tract, and urinary tract, etc. Under normal conditions, CEA is metabolized by the gastrointestinal tract, while CEA in the tumor state enters the blood and lymphatic circulation, causing an abnormal increase in serum CEA, so that serum CEA is increased in patients with various tumors mentioned above. Clinically, when CEA is greater than 60 μg/L, it is seen in colon cancer, rectal cancer, gastric cancer and lung cancer. elevated CEA value indicates residual or progressive lesions. For example, patients with lung, breast, bladder and ovarian cancers will have significantly elevated serum CEA amounts, mostly showing tumor infiltration, of which about 70% are metastatic cancers. Generally speaking, CEA level will return to normal 6 weeks after surgical resection, otherwise it indicates residual tumor. If CEA concentration continues to rise continuously or its value exceeds 5-6 times normal, it indicates poor prognosis. If the CEA concentration continues to rise or its value exceeds 6 times normal, it indicates poor prognosis. The quantitative measurement of serum CEA level in continuous follow-up is more meaningful for tumor condition judgment. It has been reported that there are heterogeneous bodies of CEA in patients with gastrointestinal malignancies, which can show 8-12 CEA peaks by isoelectric focusing electrophoresis, three of which are known to be cancer-specific peaks, called CEA-S, and the rest may belong to normal colon cross-reacting antigen clusters or other excess products of the carcinogenic process. In addition to blood, CEA quantification in other biological fluids, such as pancreatic fluid and bile, can be used to diagnose pancreatic or biliary tract cancer; CEA quantification in plasma exudate can be used as an adjunct to cytological examination; urine CEA quantification can be used as a reference to determine the prognosis of bladder cancer. Serum CEA quantification combined with thyrocalcitonin measurement can help in the diagnosis of medullary thyroid carcinoma and the estimation of recurrence.