Objective To investigate the value of MRI diffusion-weighted imaging (DWI) technique in endocrine therapy for prostate cancer (PCa) based on PI-RADSv2.1. Methods A retrospective analysis of 57 patients with pathologically confirmed PCa was conducted. All patients underwent multi-parametric MRI (mpMRI) according to PI-RADS v2.1 technical specifications before biopsy and six months after endocrine therapy. The apparent diffusion coefficient (ADC) values were measured in cancer and non-cancer areas before biopsy and six months after endocrine therapy. Patients were grouped based on the mRECIST criteria and PSA level into responders (n=45) and non-responders (n=12). ROC curves were obtained to assess the correlation between changes in ADC values and PSA values before and after endocrine therapy. Results In the responder group, the ADC value of the cancer areas was increased significantly after endocrine therapy (P<0.001). No statistically significant difference of the ADC value of the cancer areas was found in the non-responder group before and six months after endocrine therapy (P=0.714). The ADC change of responders and non-responder groups were (0.411±0.178)×10^-3 mm²/s and (-0.014±0.125)×10^-3 mm²/s, respectively (P<0.001); the ADC ratio were (60.603±30.201)% and (-1.096±13.175)%, respectively (P<0.001). The cutoff value of the ADC change was 0.165 (AUC=0.974; sensitivity, 88.89%; specificity, 100.00%; PPV, 100.00%; NPV, 70.59%). The cutoff value of ADC ratio was 16.827% (AUC=0.980; sensitivity, 91.11%; specificity, 100.00%; PPV, 100.00%; NPV, 75.00%). The ADC values were negatively correlated with serum PSA before and after endocrine therapy. Conclusion The ADC change and ADC ratio may be facilitated to monitor the efficacy of endocrine therapy for PCa. The ADC values were negatively correlated with serum PSA.