Abstract: The intensive absorption and scattering caused by seawater extremely affect the operating range and the detecting capability of underwater laser imaging technology. The absorption effect and the scattering effect of light in the seawater are analyzed. By using semi-analytical Monte Carlo algorithm, the process of how the blue-green laser transmits in the seawater is simulated. And the rules of how the photon number of the forward scatter light spot changes with the parameters, such as the average cosine of the scattered angles, the attenuation coefficient, the seawater single-scattering albedo and distance, are obtained. When the transmit distance increases, the photon amount of the forward scatter light spot will increase and tend to stable after reaching a certain distance. The photon amount has an analogous exponential downtrend when the transmit distance increases. At the same transmit distance but with different water quality parameters, the photon amount has big differences.