From the optical principle of spatial light mixer which is the core-block of the free space coherent laser communication receiver, based on it′s importance in the coherent receiver system, a structure model of the spatial light mixer is established by dielectric film polarization beam splitting (PBS) cross-spectral. Meanwhile, the mathematical relationship of optical signal and local oscillator laser effected by each optical component in the structure model is studied and the spatial light mixer mathematical model is established finally. With the mathematical model evoluted to the specific structure, the correctness and rationality of mathematical model of the spatial light mixer are evidenced by experiment. The mathematical model of spatial light mixer points out not only the relative placement of various optical components, their relative angles, but also the compensating scope of crystal phase compensation and its adjustment method, which has a great guiding significance for the development and improvement of high-performance spatial light mixer in the future.