Abstract: With an integral systems analysis computer code, the analysis model for CANDU station was built, which contains the primary heat transfer system, calandria vessel, end shield tank and the secondary sides of steam generators. The large break loss of coolant accident (LLOCA) at reactor outlet header with low pressure injection system failed and auxiliary feedwater closed was selected to make further thermal-hydraulic analysis. The accident progressions of the two loops of primary heat transfer system are different because the loop isolation valves are closed. With the failure of low pressure injection system, water in calandria vessel is boiled off and fuel bundles are heat-up. Consequently, the core is melt and collapsed into calandria vessel. At last the calandria vessel is melt-through. In the case of using injection into the calandria vessel as the mitigation measure for LLOCA, the core melting could be suspended.