Considering of orbital environments and working mode of the reflectors, thermal model of the 45-degree reflector assembly in satellite optical terminals on orbit is established to obtain temperature distribution of the reflectors(the extreme low temperature is -38.0 degrees C, the extreme high temperature is 32.2 degrees C and the max temperature gradient is 3.3 degrees C), meanwhile, a thermal-optical vacuum experiment based on the analysis results is conducted to check the effect of temperature distribution on reflectors. the results shows: if temperature of the reflector is >=-10 degrees C, the reflector shape accuracy PV will be better than lambda/3, and the RMS will be better than lambda/30 (lambda= 632.8nm), but it becomes worse and is not able to meet application requirement when the temperature decreases continuously(-40 degrees C similar to-10 degrees C). So it is necessary to design reasonable active heating loops for reflector assembly to guarantee its performance on orbit.