Abstract: Vitrinite reflectance is one of the main indexes to determine the thermal maturity of hydrocarbon source rocks. Previous results indicated that vitrinite reflectance (R_o) is not only controlled by thermal maturity, but also affected by the type of organic matter of source rock. The hydrogen content of vitrinite controls the level of vitrinite reflectance suppression and influences R_o accuracy. The fluorescence alteration of multiple macerals (FAMM) can solve vitrinite reflectance suppression effectively. The thermal maturity of 10 source rock samples from the Pinghu Formation in the Xihu Sag of East China Sea Basin were analyzed by means of FAMM and vitrinite reflectance. Results showed that the vitrinite reflectance of hydrocarbon source rocks from the Pinghu Formation were suppressed to some degree and the suppression corrected value varies from 0.01% to 0.25%. For well TWT1 the present thermal maturity of source rocks from the Pinghu Formation was determined as 0.90%-1.41%R_o by FAMM, but 0.77%-1.21%R_o by vitrinite reflectance analyses. The study helps evaluate hydrocarbon potential in the Pinghu Formation.