In order to obtain the optimal structural parameters of the absorption cavity of Solar Irradiance Absolute Radiometer(SIAR), a study was carried out on the thermal circuit of the SIAR, which was constituted of the cavity, thermal link and heat sink. Based on the finite element method by using Ansys software, a reliable model of this thermal circuit was introduced. The temperature response curves of the model were consequently obtained. Then the effects of structural parameters of the cavity, e.g. sizes and materials were investigated on the SIAR's thermal performances, and optimal structural parameters of the cavity were proposed. Simulation results indicate that at the ambient temperature of 298 K and heating power of 50 mW, the optimal parameters of cavity are silver with a wall thickness of 0.07 mm, a cone angle of 30° and a brim width of 2.2 mm, and when the thermal time constant is 11.501 s, the responsibility is 1.391 K. Experiments are carried out under the same conditions: the time constant and responsibility of measurement are 11.487 s and 1.397 K. Compared with simulation results, the differences are 0.12% and 0.43%, respectively. The simulation results are in accordance well with the theoretical derivation, and the results are sufficiently reliable, so that this model can be used to conduct the optimation design of the absolute radiometer and improve its performance.