Abstract: The incline and swing effect on natural circulation of floating nuclear power plant under site black out（SBO） accident is studied using self-developing marine condition system code RELAP5/MC. It shows that,for floating nuclear power plant under marine condition, the pressurizer fluctuating flow rate, the parallel heat sink（steam generator） have significant influences on the direct passive reactor heat removal（PRHR） system, which is different from other secondary PRHR under marine condition. The flow exchange between the loop and the pressurizer have major effect on cooling capacity for the left side loop.