All operational and under construction nuclear power plants in China are distributed in coastal areas presently. Facing the problem of spent fuel from nuclear reactors, we are also actively promoting the system construction of combined transport with highway, ocean and railway. In case of nuclear leakage accidents, radionuclides will diffuse through the ocean and pollute the ecological environment. The study of radionuclide diffusion and verification in the ocean plays an important role under nuclear leakage accidents. The hydrodynamic model and radionuclide diffusion model in coastal waters of nuclear power plant were established. The water level change and radionuclide diffusion area in coastal waters of nuclear power plant were calculated, and dangerous areas were given. The calculated results were verified by enteromorpha drift. The results show that radioactivity in the high concentration area can reach 109Bq/m3, and it is reduced by about 9 orders of magnitude to the source term. On the 14th day, the dangerous area is within about 58km to the east, 10km to the south and 21km to the west of the nuclear power plant. In the east-west direction, the dangerous area is about half of the radionuclide diffusion area.