As the non-equivalence of absolute radiometers has complex sources and it is difficult to be measured, this paper proposes the finite element method to correct the non-equivalence of a Solar Irradiance Absolute Radiometer (SIAR).On the basis of the measuring method the SIAR, the nonlinear thermal dynamic response of the cavity in vacuum was tested. A finite element model with a relative error of 0.14% for an experimental cavity was established based on the finite element method to test the temperature response of the receiving cavity. The experimental results show that the temperature difference between the receiving cavity and the heat sink is approximately 0.85 K and the time constant is 29.8 s when input optical power is 73.8 mW. The finite element method was employed to estimate and correct the non- equivalence of the absolute radiometer. It indicates that the source of non-equivalence of SIAR mainly comes from the drifts driven by different heating paths and areas, and the correctional factor of non-equivalence for the SIAR is 0.999 621±0.000 004. The finite element correct model completes the correctional system of SIAR and significantly increases the accuracy of the SIAR. It is helpful to improve the future design of absolute radiometers.