This paper describes a finite-element thermal model of an absolute radiometer designed for total solar irradiance (TSI) measurement in future satellite missions. The cavity-type absolute radiometer employs the electrical-substitution technique and active temperature control to determine the radiant power entering the receiving cavity. The nonequivalence between the shutter-open mode and shutter-closed mode due to different temperature distributions is a major factor that affects the radiometric accuracy. The analysis shows that the nonequivalence is a function of sensor positions and can be minimized by properly choosing the electrical heating method and the temperature sensor location.