This study takes the inpatient cost structure, number of hospitalizations, and length of hospital stay of prostate cancer patients as research objects. It conducts a comparative analysis of the differences in inpatient costs, number of hospitalizations, and length of hospital stay of prostate cancer patients before and after the DRG payment policy, to evaluate the regulatory effect of the policy on the consumption of medical resources by prostate cancer patients and provide data support for the optimization of the DRG policy. By collecting inpatient data of prostate cancer patients in tertiary hospitals in a certain region before and after the implementation of DRG payment, this paper uses the difference-in-differences method to evaluate the impact of the policy on inpatient costs and medical service efficiency, and applies the degree of structural variation and grey relational analysis to explore the dynamic changes in cost composition. The results show that the difference-in-differences regression model indicates that the implementation of the DRG payment reform has a significant impact on patients' drug costs, comprehensive medical service fees, diagnosis fees, treatment fees, and total costs. The Degree of Structural Variation (DSV) is 27.92%, and drug costs have the largest Variation in Structural Value (VSV) and Contribution to Structural Variation (CSV); drug costs also rank first in grey relational degree (0.9021). Therefore, after the implementation of DRG, the cost structure of hospitalized prostate cancer patients has become more reasonable, and the DRG payment reform has achieved certain results in controlling medical costs.