Abstract: For filling up the gaps in the study of beyond design basis accident (BDBA) of Xi’an Pulsed Reactor (XAPR), the reactor system was simulated by RELAP5/SCDAP/MOD3.4, and transient state operating characteristics of XAPR system under blackout accident plused anticipated transients without scram (ATWS) accident and large break loss of coolant accident (LOCA) ATWS accident were concluded. The results show that under blackout ATWS accident, a part of fuel elements will be melt without any manual intervention; under LOCA ATWS accident, the break size and location have an important influence on the result of the accident. When the break is at the bottom of pool, the highest temperature of the fuel is under 1 800 ℃. When the break is above the lower grid plate, some fuel elements will be melted. The corresponding mitigation measures were given according to the calculated results.