Abstract: The new concept molten salt reactor (MSR) is the only liquid-fuels reactor in the six candidates of the Generation Ⅳ advanced nuclear reactor, which is characterized by remarkable advantages in safety, economics, continuable development of the fissile resources, and proliferation resistance of nuclear energy. The general models of neutronics, thermal hydraulics and safety analysis were established for molten salt reactors, and the implicit method was adopted to couple the neutronics model and the thermal hydraulics model. The founded models were applied to the calculation of the molten salt actinide recycler and transmuter system (MOSART), in which the neutronics, thermal hydraulics and safety characteristics were obtained, and the influences of the inlet temperature, inlet velocity and the resistant time of the fuel salt in the external loop were analyzed.