Abstract: The technology of small modular reactor (SMR) applied to marine is getting mature gradually, but at present, the nuclear accident atmospheric dispersion researches mainly focus on coastal and inland condition. In order to qualify the atmospheric dispersion model for oceanic condition, the Gaussian plume model was modified and the consequence of small power reactor severe accident by MACCS program was studied. The results show that without plume-rise, the vertical distribution of soluble nuclide integrated concentration is greatly affected by the water and the near-sea-surface integrated concentration declines sharply. If the plume-rise occurs, the integrated concentration is low in the area near accident source term, and the influence from the water on nuclide integrated concentration is lesser. During the dispersion process, the water mainly influences the near-sea-surface area which has higher integrated concentration.