Abstract: In order to study the reactor power self-regulation schemes during transition process from forced circulation to natural circulation, mathematic models were established to simulate natural circulation transition process for an integrated marine reactor. The results show that during transition process, the core inlet and outlet coolant temperatures have a strongly delayed effect under natural circulation conditions, and the coolant temperature weights are high in the nuclear power self-regulation model. Thus the measuring point positions of the core inlet and outlet coolant temperature have a significant influence on the peak value of key parameters. The closer to the core the measuring point positions of core inlet and outlet coolant temperature are, the smaller the peak values of key parameters are during the transition process, the shorter the transition process time is and the control process has a better effect.