Abstract: Accident is one of the basic properties of PWR. Among these accidents, the large break loss of coolant accident is one of the accidents with complicated phenomena and severe consequences which may result in nuclear accident. The paper focused on a small power reactor, analyzing and calculating the source term in-containment separately in 50, 150, 320 days full power operation, with cold leg or cold and hot legs emergency core cooling system (ECCS) injection, with or without emergency ventilation conditions. Some of the results were compared with those of safety analysis report. The results show that the assumptions are reasonable, the results are logical. It is important for safety operation and nuclear emergency of nuclear reactor.