CA125, also known as glycoantigen 125, is mainly used to detect tumor recurrence and metastasis, assess the effectiveness of treatment, determine prognosis, and is an auxiliary diagnosis for many tumors. If the value of CA125 increases by ≥2 times the reference value, it should be considered a serious condition. This antigen is also present in trace amounts in normal humans, such as fetal corporal epithelial secretions, maternal amniotic fluid, adult fallopian tubes, uterus, and endocervical lining. When CA125 is persistently and progressively elevated, the higher the probability of developing tumors, such as stomach, breast, ovarian, colorectal and lung cancers, CA125 will be abnormally high because this antigen is mainly distributed on the surface of cancer cells, which can block cell differentiation and thus promote the development of cancer cells. However, if this antigen is the only marker that is elevated, it cannot be used as evidence of the existence of tumor. One should immediately consult a doctor to improve the pathological examination and needs to be combined with the patient’s specific medical history, clinical manifestations and other auxiliary examinations. If there is cancer cell activity in human body, CA125 will increase abnormally. Generally speaking, CA125 level will return to normal after 6 weeks of surgical removal of the affected cancer lesion, if it does not drop, it indicates that there may be residual cancer cells, if the value continues to rise and once exceeds 5-6 times the normal value, it may indicate malignant tumor with poor prognosis.