Abstract: Considering the fuel salt flow effect of the molten salt reactor, the neutron dynamic model was derived based on the particle conservation equations, and the critical problem was calculated by numerical method under three working conditions to study the flow effect to effective multiplication factor, fast neutron, thermal neutron and delayed neutron precursors distribution. The calculated results show that the influence of mass flow on the effective multiplication factor is very small, and the influence on the thermal neutron distribution is larger than that on the fast neutron distribution. In addition, the percentage of the delayed neutron precursors moving out the core increases with the increasing mass flow.