Abstract: Loss of coolant accident (LOCA) is one of the most important accidents in PWR, and total water loss is closely related to the severity of accident consequences. The NHR-200Ⅱ is a full-power natural circulation low-temperature nuclear heating reactor designed by Institute of Nuclear and New Energy Technology of Tsinghua University with many years of research and continuous improvement. Its design adopts a variety of advanced passive and inherent safety designs. For the NHR-200Ⅱ reactor, the typical pipe rupture of hydraulic control rod driving system with isolation failure was selected to be simulated and analyzed by the thermal-hydraulic transient analysis program. The results show that the residual coolant in the primary circuit of NHR-200Ⅱ can cover the reactor core all the time even in the most serious loss of coolant accident, and the heat of reactor core could be carried away effectively by the passive decay heat removal system, which ensure that the reactor core would not be damaged due to lack of coolant. The result indicates that the NHR-200Ⅱ has good safety characteristics.