Freeze valve is an important passive safety system used in the molten salt reactor (MSR). In the freeze valve, salt is frozen by external cooling in the pipe connecting the reactor core and drain tanks. On the occasion of station blackout, the cooling system stops and the frozen salt melts automatically to open the freeze valve. Then, the molten salt containing fuel is drained out from the reactor core to the drain tanks and the nuclear reactions stop passively. Obviously, the melting time of frozen salt is important from the viewpoint of MSR safety. In this study, experiments and numerical simulations were conducted to investigate the effects of important parameters on the opening time of freeze valve. A particular attention was paid to the effect of pipe wall since its thermal conductivity is much higher than that of salt. The calculated opening time agreed with the experimental results fairly well, indicating that the present numerical model is useful to investigate the opening time of freeze valve in accidental condition of MSR.