Abstract: Using the model of Gaussion subsection plume, the radioactive nuclide atmospheric dispersion rule in the terrain of 20 km of the coastal was estimated when the design basis accident with 29.4% equivalent diameter break size happened. The source term was captured by the calculation program of severe accident named MELCOR, and the result was used as input in the analysis software of atmospheric dispersion named MACCS. The results show that the mid-break LOCA would lead to the radioactive pollution for the area of dock. The slower the wind blows and the more steady the weather is, the larger the radioactive polluted area is.