Combined with Sahu-Shanmugam and Fournier-Forand volume scattering functions, a simulated model of underwater laser transmission channel is built with Monte Carlo method, and the model is used to analyze the beam extension characteristics at the receiving end. The effects of received field of view and the diameter of receiver on the power density of the beam, and the distribution characteristics of beam power density under different receiving distances are studied under three typical waters. The results show: with the increase of scattering coefficient in the water area and the increase of transmission distance, the beam distribution expansion is intensified; with the increase of diameter of receiving surface, the variation trend of the beam power density decreases gradually, and the amplitude of the beam power density increases as the increasing receiving field angle; with the increase of transmission distance, the distribution of beam power density becomes more discrete. The results provide a reference for underwater positioning or underwater receiver design.