Abstract: Based on the basic principle and structure of passive nitrogen-typed accumulators, the thermal-hydraulic model was built up and the transient analysis code for accumulator of AP1000 (TACAP) was developed. In addition, the performance of the accumulators was analyzed during different cold leg small break loss of coolant accidents, including 25.4 cm equivalent diameter cold leg break and 5.08 cm equivalent diameter cold leg break. This study also brought about some key transient characteristics during these accidents, such as water level of tank and injection flow rate. The calculation result shows that the injection flow of accumulator can effectively lower temperature and reduce pressure of primary circuit. The calculation result of TACAP also shows good agreements with the calculation result performed by Westinghouse (USA) with NOTRUMP code, which demonstrates the applicability and the validity of TACAP.