Abstract: As offshore floating nuclear plant (OFNP) operates in the marine environment for a long time, all systems will be affected by the ocean motion conditions. Therefore, it is necessary to consider the influence of ocean motion conditions in the design, operation and accident safety analysis of OFNP. Station blackout (SBO) accident has been an essential part of nuclear power plant accident safety analysis after the Fukushima nuclear accident and necessary safety systems must be established in nuclear power plant to mitigate or delay fuel degradation during SBO accident. The passive residual heat removal system (PRHRS) is an important auxiliary system in nuclear power plant, which can bring out the residual heat and prevent the core melting after the SBO accident. However, current researches focus on the following aspects: 1) the development of system analysis code suitable for ocean motion conditions; 2) the analysis of natural circulation characteristics under ocean motion conditions; 3) the computation and analysis of single kind of PRHRS operation characteristics. While little research work has been carried out to compare the performance of PRHRS in primary and secondary circuits under static and motion conditions. In this paper, a system analysis code applicable to OFNP was developed by adding motion condition model to existing code and validated by comparing to experimental data, furthermore an OFNP system model with PRHRS in primary and secondary circuits was established and SBO accident simulation and safety analysis were conducted under static and rolling conditions with PRHRS in primary and secondary circuits operating respectively to compare the performance of the two PRHRS. The results show that the modified code shows great accuracy and feasibility by conducting simulation of a twoloop single phase circulation system and comparison of simulation results and experimental data. The heat removal capability of PRHRS in secondary circuit is better than that in primary circuit under static condition while both of the two PRHRS can effectively extract the residual heat from reactor core after the SBO accident. The volume of secondary PRHRS heat exchanger can be smaller when the heat removal capability of the two PRHRS is the same. Rolling motion condition has little influence on both of the two PRHRS, and PRHRS in the secondary circuit is more susceptible to pitching motion condition. Since secondary PRHRS is more susceptible to motion conditions, it needs to be selected carefully when reactor system operating in motion environment is designed and evaluated.