Internal flooding is a major risk threatening the safety of nuclear power plants. Internal flooding is a common internal hazard of nuclear power plants. The flooding effect of internal flooding may lead to abnormal operation of nuclear power plant equipment. Nuclear safety equipment fails to perform related security functions. Nuclear power plant safety is threatened. 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. There is no comprehensive internal flooding safety assessment and analysis practice in domestic small modular reactor. However, in the design process, internal flooding safety assessment of small modular reactor must be carried out. This study summarizes the research and practical experience of the regulations and standards, this method concludes the consequences of systems and equipment influenced by internal flooding based on deterministic theory. This method can find out the weakness of the plant, improve its design and increase the economy. At the same time, based on the actual scenario of the project, this study selects several typical flooding areas for safety assessment, and verifies the applicability and operability of the internal flooding deterministic safety assessment method for small modular reactor.