Abstract: In this paper, Geant4 was used to simulate the process of underwater radioactive source generating Cerenkov light and transportation of Cerenkov light in water, and the intensity of Cerenkov light and distribution of photon number in the horizontal plane were obtained. The theoretical verification and the feasibility analysis of this technical means were performed. Calculation results show that when the energy of emitted particle is greater than the threshold energy of Cerenkov light generating, the Cerenkov light spot as well as the distribution of Cerenkov photon number on the surface of water is regular, and then the location and depth of radioactive source can be deduced. Thereby, this work contributes a theoretical basis for the development of using Cerenkov light to search underwater radioactive source.