Abstract: For structural and operational characteristics of marine nuclear power platform, a transient analysis model of the primary loop was developed which considered the swing impact of the marine nuclear power platform. According to the results of the code, the swing state will adversely affect the natural circulation. In case of a 45-degree swing state, the natural circulation mass flow rate will drop about 5%. Improving the height of the steam generator above the reactor core and reducing pump resistance can effectively decrease the temperature of the core for natural circulation state and improve the natural circulation mass flow rate.