In the experiments of laser signal transport in rain medium, it is obvious that the transmission in heavy rain is bigger than that in light rain. This shows that the attenuation of light signal in light rain is bigger than that in heavy rain. The transmission attenuation of wireless laser communication in rain is noticeable. According to the theory of Mie and Weibull raindrop spectrum, the effects of the particle scales on laser scattering and attenuation efficiency factor are analyzed, and the attenuation formula of the light wave of a single particle is deduced. The definite equations between the attenuation and the rainfall ratio are obtained. The result shows that scattering intensity of particle in heavy rain is bigger than that in light rain in forward direction, and the forward scattering intensity increases. When laser signal transmits through rain, attenuation coefficient in light rain is big, while it is small in moderate rain and heavy rain, and increases in rainstorm. This result coincides with the facts. Attenuation characteristic of laser in rain provides theoretical basis for laser applied in the communication system working in the rain.