Because of the scattering and absorption of raindrops, laser power attenuates when travelling through the rain. Firstly, according to the Mie theory, the method of calculation of the extinction factor in the rain was presented. Secondly, Joss and Weibull raindrop size distributions were introduced, and the comparative analysis was shown that Joss raindrop size distribution was more precise. At last, the extinction factor and link loss of long wavelength infrared laser in the rain were analyzed by using Matlab. From the analytical results, it is cocluded that the value of the extinction factor becomes larger with rainrate increasing; Considering beam divergence, field of view and scattering energy, the link loss calculated by photon tracing method is more accurate compared with Lamber-Beer Law.