Abstract: In order to better understand the startup characteristics of secondary passive residual heat removal (SPRHR) system specifically for the second generation+ 1 000 MWe PWR nuclear power plant in China, a study was done on startup response of the system and potential steam hammer. The results show that with appropriate time step and mesh length, RELAP5 is able to model the steam hammer which occurs within SPRHR system. Different startup strategies lead to significant differences on the system’s response. This study suggests with appropriate startup procedure and startup speed, the steam hammer could be effectively weakened or even eliminated, and the startup stability of the system could be greatly improved.