It is used for quantitative detection of free prostate specific antigen (f-PSA) in human serum in vitro. Free prostate specific antigen (f-PSA) is a serine proteolytic enzyme secreted by prostate epithelial cells with molecular weight of 34000. About 80% of free prostate specific antigen (f-PSA) in Serum combines with some protease inhibitors to form complex (CPSA). 20% of PSA exists in unbound form, which is called free PSA (f-PSA). In the screening and detection of prostate cancer, when the low level of total PSA increases, the level of f-PSA does not increase in proportion, and the ratio of f-PSA / PSA is less than 1 / 10, the probability of diagnosis of prostate cancer increases significantly. In prostate cancer, the normal structure of glandular duct is destroyed, and the PSA content in serum is increased. The ratio of f-PSA to PSA is mainly used in the differential diagnosis of benign prostate disease and prostate cancer. The kit is used for the auxiliary diagnosis, efficacy evaluation and dynamic monitoring of disease progress of prostate patients.