Abstract: In order to ensure the safety functions of nuclear power plants in case of internal flooding, it is necessary to consider protection against internal flooding in nuclear power plant design and evaluate the consequence to verify that the target of internal flooding protection can be achieved. According to the study on internal flooding protection purposes, requirements and protection measures, assumptions, methods and steps of internal flooding deterministic safety assessment were sought and presented. Based on a boric acid transportation room and reactor cavity and spent fuel pool cooling and treatment (PTR) pump room of 1 000 MWe nuclear power plant, the method was applied and verified. The analysis result shows that the boric acid transportation room doesn’t need any protections, but the PTR pump room needs to take drain measures in order to perform the nuclear safety functions.