Abstract: A tightly coupled multi-physics model for molten salt reactor(MSR) system involving the reactor core and the rest of the primary loop has been developed and employed in an in-house developed computer code TANG-MSR. In this paper, the computer code is used to simulate the behavior of steady state and transient state of our redesigned thorium molten salt reactor(TMSR). The simulation results demonstrate that the models employed in TANG-MSR can capture major physics phenomena in MSR and the redesigned TMSR has excellent performance of safety and sustainability.