Underwater light field distribution has a decisive impact on the underwater photoelectronic imaging quality, and the transmission degradation model of underwater images can be obtained through the underwater light field distribution. Giving the optical parameters of water body, the probability model of underwater photon transmission was established. By using the Monte Carlo method to simulate the underwater photon motion, the volume density distribution of the underwater photon scattering points could be obtained and the underwater light field could be solved. Under the condition that the water volume scattering function was spherical symmetry, the underwater light field distribution solved by Monte Carlo method was in good agreement with the results of the theoretical formula, which verified the effectiveness of this method. Finally, the distribution of underwater light field formed by underwater ideal point light source was given. Based on this conclusion, the application range of Monte Carlo method in solving underwater light field can be extended to the case when the water volume scattering function is a general expression.