Weather conditions and atmosphere have greater influence on the free space Atmospheric Laser Communication (ALC) between the ships. However, most of the research work focus on the discussion on the scattering attenuation of laser beams caused by atmospheric effects. This paper points out that the free space laser communication in raining is influenced by the blotting-out effect of the rain drops mainly, especially the overlapping effect between the projecting areas of rain drops on a receiver. By which, the phenomena that the ALC is achieved within a certain distance in heavy rains is well explained. Based on the raindrop size distribution, a mathematical model of the attenuating effect of rain's blotting-out on the ALC is presented, and the transmitting distance of ALC and the attenuation of the light intensity are calculated. The results show that the model is reasonable and results of the model is coincided basically with that of the experiments. The obtained results provides a basis for research of the performance of ship free space laser communication.