The invention provides a kit for detecting PD-L1 gene mutation of peripheral blood circulating tumor cells of a prostate cancer patient and a detection method. The kit comprises a diluent, a destaining , a staining solution A, a staining solution B, an anti-human PD-L1 primary antibody, an enzyme-labeled goat anti-human secondary antibody, 0.1% of Triton X-100, 0.3% of H2O2 and a reagent A. The detection method comprises the following steps: separating and acquiring peripheral blood circulating tumor cells (CTC) by using a membrane filtration device, and detecting the PD-L1 expression condition of the peripheral blood CTCs by using an immunohistochemical technology. By utilizing the detection method provided by the invention, the PD-L1 expression condition of a patient with advanced or recurrent hepatocellular carcinoma can be detected without acquiring a tissue specimen through needle biopsy, and real-time dynamic detection can be realized by utilizing a minimally invasive technology;by adopting the separation device and the diluent, the circulating tumor cells are well separated, the interference of blood cells is avoided, the false positive result caused by the edge effect possibly generated in the dyeing process is avoided, the stability is good, the cell loss is reduced, and the detection accuracy is improved.