This work employs finite element (FE) software to perform a thermal modeling of a proposed absolute radiometer designed for the future satellite mission of total solar irradiance measurement. Both steady-state and transient analyses have been performed to obtain the temperature distribution and history. 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 steady-state 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. The transient analysis provides the temperature response for a step power input. In order to reduce the computational time, simplified lumped-heat-capacity models have been developed by applying the least-squares fitting technique to the transient result of the FE model. The two-lumped-heat-capacity model demonstrates a better accuracy than the single-lumped-heat-capacity model and will facilitate the controller design.