Abstract: With wide application of nuclear techniques, accidents of lost radioactive sources increase. The airborne gamma spectrometer can be used for searching the lost radiation sources on the ground level. However, for radioactive sources lost in water, the use of gamma spectrometer is limited as a result of the shielding of gamma rays by water. So detection of underwater radioactive source based on Cherenkov light generated by the radioactive source is becoming important. With applications of combined simulation of Geant4 and MCNP, and continuation simulation method in Geant4, distributions and transmission of Cherenkov light generated by underwater Co-60 sealed source were simulated. The simulation reveals that wavelength of Cherenkov light is between 300~600 nm through transmission in water. The light intensity becomes stronger from the edge to the center, and the distribution range approximately equals to the depth of the radioactive source in water. The light flux is about 100 Cherenkov photons·cm^-2 after 300 m transmission in water. The Cherenkov light can be detected by the characteristics of its wavelength spectrum and intensity distribution.