Abstract: The influence of outside disaster to the nuclear power plant (NPP) is paid more and more attention after Fukushima accident, and internal flooding is one of the important and common disasters that may induce the severe accident and need to be analyzed. In this paper the effect of design and operation on NPP was discussed based on the study of flooding probabilistic risk analysis, and the contribution for internal flooding risk evaluation was put forward. The results show that electrical building is the most contribution building to the core damage frequency (CDF) induced by internal flooding, and equipment cooling water system is the most contribution water source to the CDF induced by internal flooding. Flood protection, detection and alarm measures in the related area are needed.