Abstract: Compared with solid fuel reactors, there are differences in physics for liquid fuel reactor. As for molten salt reactor (MSR), due to fuel flow in primary loop, the delayed neutron precursors and fission product partly decay out of core, resulting in reactivity loss as well as heat generation in the primary loop. In this paper, the critical dynamics and safety characteristics of MSR were investigated using Cinsf1D code. Considering the loss of delayed neutrons under different fuel flow speeds at zero-power, the corresponding control rod positions were calculated under pump start and stop conditions. Keeping reactor power at 2 MW, the temperature and power were computed for the primary loop system. Finally, the pump stop accident was simulated from rated power 2 MW. After pump stop, the reactor power increases slightly due to the reduction of delayed neutron loss at initial time and then it decreases to approach the decay heat power level quickly. The temperature in core increases slowly and reaches to a balance within safety range. It can be concluded that MSR has intrinsic safety.