To ensure the stable operation of spaceborne coherent laser communication systems based on polarization optical path under extreme space thermal environments, a thermal control design scheme is proposed. This scheme combines passive and active thermal control technologies. The external heat flux distribution under typical orbital conditions is analyzed through NX/TMG software simulation, leading to the determination of a passive thermal control strategy involving multilayer insulation material wrapping and heat dissipation surface optimization. Additionally, a zoned temperature control active heating system is designed, utilizing a proportional-integral-derivative（PID）algorithm to achieve a temperature control accuracy of ±3℃. Thermal balance test results show that the optimized system maintains polarization purity stably above 95%, with beam divergence angle controlled within 65±5 μrad under 22 ±3℃ conditions, significantly enhancing the thermal stability of the optical system and communication reliability.