Abstract: In order to accurately evaluate the level of marine radioactive pollution caused by airborne nuclides released after a serious nuclear accident at sea in a small power reactor, a severe nuclear accident of the small power reactor induced by the power failure is taken as an example. A calculation model of nuclide diffusion in the atmosphere and the ocean was established to calculate the level of 137Cs radioactive pollution in the atmosphere and the ocean after the accident. The effects of airborne nuclide release height, atmospheric stability and source release ways on the marine dispersion of the nuclides were preliminarily discussed. The results indicated that at a certain release height, the time integrated concentration of nuclides in the surface seawater on the downwind axis increases with the distance, and then decreases after its maximum. Within a certain distance to the source, the higher the release location is, the smaller the time integrated concentration of the nuclides in the surface seawater is. When the distance from the source is greater than a certain distance, the more unstable the atmosphere is, the smaller the time integrated concentration of nuclides in the surface seawater is.