本文用蒙特卡罗模拟方法，研究了用于探测水下目标的海洋激光雷达系统中光束的扩散，得到了各种水质条件下不同深度处水下光斑的半径，结果与国外的有关研究结果相吻合；我们还首次计算了水面上目标返回光斑的半径以及其中的亮度分布，并研究了海浪和泡沫对光束扩散的影响，这些结果对于海洋激光雷达系统的设计具有非常重要的意义。 In this paper, the Monte Carlo simulation method is used to study the diffusion of light beams in a marine lidar system for detecting underwater targets. The radii of underwater spots at different depths under various water quality conditions are obtained. The results are compared with the results of foreign studies For the first time, we also calculate the radius of the target return spot and its brightness distribution on the water, and study the influence of waves and bubbles on the beam diffusion. These results are of great significance to the design of the marine lidar system.