Tumor markers are chemicals that reflect the presence of tumors. Some tumor markers exist only in embryonic tissues, while some tumor markers exist not only in tumor tissues but also in normal tissues, only that their levels are much lower than those of tumor tissues. In clinical practice, we mainly base on their dynamic changes and also combine with other tests to diagnose tumors, judge prognosis and guide treatment. The sensitivity of tumor markers is not high, and false-positive and false-negative results occur in clinical practice, so we cannot confirm the diagnosis of tumor simply by tumor markers. If the test result is persistently elevated and substantially higher than the normal range, it indicates the possibility of tumor tissue existence, which should be combined with related auxiliary examinations (such as X-ray, B-ultrasound, CT, MRI, bronchoscopy, gastroenteroscopy, etc.) for diagnosis. Pathological examination is the gold standard for tumor diagnosis, which is the basis and fundamental of all subsequent treatments.