Abstract: In order to investigate the pressure distribution outside the vessel during the process of steam explosion including both the premixing stage and explosion stage,a three dimension multiphase fluid analysis code MC3D has been used based on the structural and environmental characteristics of the sever accident state of 1000 MW level PWR Ling’ao Nuclear Power Station Phase II with the consideration of both pressure variation and impulse variation.The results indicate that the process of premixing has a low level pressure variation with a long duration.On the contrary,the explosion stage creates a high level and sharp pressure peak which threatens the integrity of the reactor cavity structure.In addition,the mass of the melt-drop con-tacts the coolant all over the calculating cells has a significant effect regarding the magnitude of the impulse created by the steam explosion.