Abstract: Passive safety systems will be affected by ship motion and the introduced additional inertial force which varies with the time and space as it operates in marine conditions. To study the transient operational characteristics, a mathematical model of the Passive Residual Heat Removal System (PRHRS) was established and a simulation program developed. Six degrees-of-freedom motion was investigated respectively, and the results show that the natural circulation capacity of PRHRS decreases and flow rate oscillates due to marine conditions, while the rolling motion has the greatest impact. However, the swaying and heaving motion has no effect on the average flow rate of PRHRS though the observed flow rate oscillation. In addition, the results also show that the PRHRS system can start normally and remove the residual heat from the nuclear reactor within the calculated parameter region.