Abstract: In this paper, floating nuclear power plant is studied and RELAP5 is used to analyze the effect of water-seal-structure on the safety injection when occurring the lose of coolant accident with a particular broken size, and investigate the influence pattern of water-seal-structure on the safety injection. The result shows that: the water-seal-structure is harmful to the safe injection of water into the reactor effectively which may lead the reactor to a danger state. This harmful influence can be controlled by increasing the pressure and flow rate of safety injection, which give rise to the increasing demand of system capacity. According to the design features of floating nuclear power plants, the primary passive residual heat removal system can bypass the water-seal-structure after LOCA,which can guarantee the effective injection of water and auxiliary removing the decay heat of the reactor. The operation mode is conducive to the optimization of the capacity design of safety injection.