The laser fuze working in the rain would be scattered and attenuated by the raindrops, so that the beam quality will decrease and echo power reduce, consequently affect the function of detecting system. Based on Monte-Carlo method, the transmission model of laser in the rain is founded, and beam spread characteristics at different simulation conditions are analyzed, and the model is validated by experiments. The simulation results indicate that the raindrop scattering induced to beam spread and power density decreasing, and along with the augment of rainfall rate, transmission distance and beam divergence, the photon scattering times increase and spot diameter extendes, the photons arriving at the receiving surface decreases and beam power attenuates. Experimental results are consistent with the simulation calculations and the largest error is not more than 4%, which greatly agree with the model.