Abstract: The safety characteristic of the reactor core, in which the inadvertent control rod withdrawal accident happens, was simulated and analyzed by the three-dimensional safety analysis and simulation software of the ship reactor. When one group or bundle of control rods in the reactor that was operating under the high or power condition were inadvertently withdrawn at the different velocities, variations of important reactor parameters, such as the peak temperature in the fuel center, the minimal departure from nucleate boiling ratio (DNBR) and the outlet coolant temperature, et al, were investigated. Some useful conclusions are drawn, which are very significant and important to estimate the reactor safety state and to provide safety operation rules for the reactor operator after the accident.