Abstract: The steam generator tube rupture (SGTR) accident of a marine nuclear reactor was analyzed, and mathematic model for the radionuclides’ transport and leak was built. Coupled with the thermal-hydraulic results, the radioactivity in a certain cabin of the marine nuclear facility was got. The mathematic model is helpful for radioactivity analysis and protection. The mathematic model has been already used in the radioactivity analysis platform for typical accidents of the marine nuclear reactor.