Abstract: the MCNP code is used to simulate the radiation environmental conditions of different regions of the power plant at different periods under a postulated accident. The results indicate that peak dose level appeared about 2.5 h after the release of the nuclides in the NPPs areas, and thereafter, the dose level gradually decreases as the decay of the nuclide proceeds; in addition, the areas near the ECCS equipment and pipelines maintain an extremely high dose rate for a long time after accidents, and the expected personnel accessibility problem will produce a threat to accident mitigation.