Abstract: Abnormal proliferation or aggregation of marine organisms is the major factor affecting the safety of water intake systems for coastal nuclear power plants. Screening and risk assessment of blockage-causing marine organisms are essential for ensuring the effectiveness of early warning and prevention of water intake blockages. This study, based on the characteristics and safety requirements of the cooling water system in coastal nuclear power plants, employs the Analytic Hierarchy Process (AHP) and Biological Traits Analysis, and selects eight indicators including individual/group size, softness, escape ability, clustering tendency, habitat density, biomass, outbreak history, and qualitative interview from three levels (biological characteristic, biological quantitative index, and qualitative index), based on the biological and ecological characteristics with expert opinions and feedback from operating nuclear power plants. This study determines the weight, grading assignment, calculation method, and classification criteria for the evaluation indicators were determined, establishing a risk assessment index system for marine biological blockage in coastal nuclear power plants. And applied our evaluation method, the risk of blockage-causing marine organisms was predicted and evaluated using data from multi-voyage marine survey. The predicted results were consistent with the validation data of filter backwashing, indicating that the risk assessment index system for blockage-causing marine organisms in coastal nuclear power highly feasible and accurate, and can be further promoted and applied in more nuclear power plants and areas in the future.