This clinical study is an open-label, single-center, observational study to evaluate the application value of customized ctDNA monitoring in efficacy assessment and prediction during PARPi treatment in mCRPC patients. A total of 30 participants with second-line treatment failure will be registered in this study. Whole blood collection will be conducted during the treatment for ctDNA detection, homologous recombination repair (HRR) genes testing, personalized panel customization and whole exome sequencing.