The transmission process of LD pump light in the water cooling structure of side pump module is studied in this paper. Based on the influence of module structure parameters, light wave transmission characteristics and medium transmission characteristics, the polarization state change, light intensity attenuation process and change rules of pump light on the surface and inside of each medium are systematically analyzed. The attenuation of light intensity caused by the surface Fresnel effect and internal absorption effect is mainly analyzed. Finally, the loss coefficient and polarization state change matrix of the pump light in a single round trip in a water-cooled structure are calculated. The analysis results can be used for reference to optimize the pumping efficiency and enhance the heat dissipation effect of the module.