The scattering of light by water is an important factor in the deterioration of underwater image quality. In order to quantitatively analyze the influence of water scattering under the irradiation of a specific light source, the scattering model of underwater light transmission is established, and the Fredholm integral equation for solving the distribution of underwater light field is derived. Under conditions where the light energy underwater decays exponentially with an increase in distance and the volume scattering function of the water is constant, the numerical iterative solution method of the integral equation with boundary conditions is given and the high-precision underwater light field distribution can be obtained. Taking the sun, uniform sky brightness, and underwater and overwater point light sources as examples, the calculation results of their underwater light fields when the water surface is calm are given. This method can be extended to solve the distribution of underwater light fields under arbitrary light source configurations and arbitrary boundary conditions, which lays a foundation for strictly deriving a point spread function and modulation transfer function for water bodies.