Abstract: The performance of RCP hydrodynamic shaft seals in both three positions degraded near the end of operation cycle, which is the feedback from NO.103 overhaul in one Chinese domestic nuclear power plant. The low-pressure leakage is above normal level, and the pressure before No.3 seal face decreases by 15%, according to the field monitoring data. In order to find out the reason of this phenomenon, a full-scale test rig was set up. An optimal design of the secondary seal is applied by Optimization the dimension of the O-ring. The friction forces of the secondary seal couple are cross measured. The No.3 guide ring is updated by coating with Cr3C2. The results show that the friction force of the original secondary seal pair is above 300 N, and the friction force of the new design is reduced to less than 100 N. The seal leakage was reduced by 11.7%. After that, a total 500 hours’ endurable experiment is done. The sealing performance data and No.3 guide ring surface roughness value change were recorded before and after the experiment. The test data proves that the optimized design can improve the RCP shaft seal reliability.