Abstract: A typical three-loop pressurized water reactor (PWR) as reference planted is chose, and the PWR severe accident analysis models based on RELAP5/SCDAP/MOD3.2 code are developed. The best estimate calculation of hot leg large break LOCA of 18 cm without mitigating measures is performed. The mitigating capability of injection to the accident is studied when peak core surface temperature reaches 1100 K, 1300 K and 1500 K respectively. The results show that the injection occasion plays an important role in the accident progression. Injection during early stage will prevent core from melting effectively. However, injection during later stage may deteriorate the progression of severe accident and accelerate of core melt.