Abstract: In normal operation of coastal nuclear power plant, seawater is extracted as the circulating cooling water. The phenomenon that the strong flow of cooling water entrains small marine plankton, fish eggs, larvae and juveniles into the condenser cooling system through filtration facilities is called the entrainment effect. At present, the loss of marine life caused by the entrapment effect is evaluated according to the algorithm mentioned in the technical specification for the impact assessment of construction projects on marine living resources（SCT9110）. Based on the basic rules of nuclear power plant operation, considering the non-operation period of shutdown and refueling and the seasonal distribution of the marine plankton, an optimization algorithm for the loss of the marine plankton in water intake is proposed. By comparing with the original algorithm, it is confirmed that this method can more comprehensively and objectively analyze the marine life loss based on the water intake volume.