Abstract: The natural circulation capacity is an important parameter that characterizes the safety capability of a nuclear reactor.We calculated the transient characteristics under the conditions with and without emergency cooling system by RELAP5/Mod 3.4, analyzed the transient natural circulation capability of the coolant flow direction reversal process, and acquired the maximum transient transfer capability.The results show: the emergency cooling system reduced fuel and coolant temperatures significantly and improved the safety of the reactor; when the decay power was reduced to 590 kW, the auxiliary pump was turned off and the reactor could be brought into a safe state with natural circulation.