Prostate-specific antigen (PSA) is a glycoprotein synthesized by prostate tissue and secreted by prostate epithelial cells with a relative molecular weight of (33-34)X103 . About 85-90% of PSA in serum is bound to protease inhibitors, called total PSA (T-PSA), and about 10-15% of PSA is in free form, called free prostate-specific antigen (F-PSA), which is present in minute and constant amounts in serum. The amount of the latter is minimal and constant in the serum. PSA has target organ specificity, and the concentration of PSA in the serum of prostate cancer patients is significantly higher, so the determination of serum PSA is now recognized as the preferred marker for the diagnosis of prostate cancer. The sensitivity of PSA is between 40~75% in early stage and up to 75~95% in middle and late stage, so it has a certain clinical value for the diagnosis and disease course monitoring of prostate cancer. 1. Screening test for prostate cancer: The PSA concentration distribution between prostate cancer and benign prostate disease has a certain overlap (4-10ug/l), so PSA measurement alone in the early stage of prostate cancer has little diagnostic significance, if combined with F-PSA test, the ratio of F-PSA/ T-PSA has significant value in determining the nature of prostate tumor. If F-PSA>25% suggests a benign prostate lesion, if F-PSA<25% will have a high risk of prostate cancer. 2. Measurement of PSA before treatment: The level of serum PSA before treatment is negatively correlated with the likelihood of successful treatment. The diagnosis of organ-limited prostate cancer depends on the PSA value. 70% of patients with pre-surgical PSA levels <2 ug/L have disease confined to the organ, while about 82% of patients with PSA levels >50ug/L show absolute indication for surgery or have pelvic lymphatic metastases. 3.Monitoring of disease course after prostate eradication: PSA cannot be detected within 3-6 months after surgery, and if PSA concentration is elevated it predicts the presence of residual prostate cancer tissue. 4.Monitoring of disease course after radiotherapy: half-life of PSA >88 days after radiotherapy is a sign of tumor progression. If PSA level decreases to the reference range within 12 months is a sign of better prognosis, while not decreasing to <4 ug/l, there is a high risk of progressive disease occurrence.