Abstract: To study the effect of ocean conditions on the station blackout (SBO) accident process for floating nuclear reactor and the response characteristic of passive safety system, a modified RELAP5 code which considered ocean conditions was developed and validated by experiment. A calculation model of two-looped floating nuclear reactor with the secondary passive residual heat removal system (PRHRS) was established. The numerical analysis of SBO accident in floating nuclear reactor under different rolling conditions was conducted by the modified code. The results show that rolling motion accelerates the decreasing rate of system pressure and temperature. The core decay heat can be effectively removed by the secondary PRHRS. But ship rolling motion can cause the marked descent of the natural circulation flow rate in the core during SBO accident process. The natural circulation flow rate of the primary loop and PRHRS oscillates with rolling motion and periodic reversal flow appears. The results can provide technical support for the design of passive safety system of floating nuclear reactor.