Abstract: Coolant temperature and velocity fields of rectangular channel in a marine reactor were simulated and analyzed with computational fluid dynamic codes when the ship was swinging. Effect of the mass flow rate distribution variation by ship motion on channel inlet average velocity was taken into account. The result shows that during the ship’s swinging, coolant temperature and velocity fields are affected by the variation of inlet average velocity and swinging of the channel because of the ship motion. When the variation of channel inlet average velocity is great, the change of coolant temperature and velocity fields is dominated by it. But while it is little, the change of coolant temperature and velocity fields is affected by the channel swinging remarkably.