Abstract: A steady temperature field mathematical model of complicated construction satellite is established in the paper based on Monte Carlo-Domain Decomposition(MC-DD) method. The sun incident areas, infrared and sun radiation transfer coefficient of surfaces are numerical simulated. The temperature distribution in different sun irradiation conditions and its variation effected by non-uniform and instability of sun irradiation in solar simulator are calculated and analyzed. The results show that it is an effective means extremely for simulation study on space outer-heat flux simulated by solar simulator, and the effects of temperature distribution by non-uniform sun irradiation is greater than the instability.