The thermal load of the cryogenic infrared detector Dewar is a comprehensive indicator characterizing the adiabatic capacity of the Dewar. Radiative heat is a part of the thermal load. When calculating the radiative heat ,the traditional approach typically simplifies the Dewar to a coaxial cylindrical model. This simplified model differs significantly from the actual one and the traditional approach is incapable of computing the radiative heat transfer between surfaces where emissivity, transmittance, and reflectance vary with wavelength. To enhance the calculation accuracy of the Dewar's radiative heat, based on the Monte Carlo principle, a 3D Studio Max modeling was employed, model information was extracted, and a program was developed, resulting in a set of general calculation programs for the Dewar's radiative heat based on the radiation transfer factor. To preliminarily verify the accuracy of the calculation program, the cold side radiative heat of two types of experimental Dewars was calculated under the gray body assumption and the errors between the calculated and experimental values were 19 mW and 8 mW. After the initial verification of the calculation program's accuracy, considering the variations of the material surface emissivity, transmittance, and reflectance with wavelength and the influence of temperature on the radiation wavelength, the cold side radiative heat of a typical 1K×1K long-wave Dewar for engineering applications was calculated. The error between the calculated and experimental values was 17 mW. Finally, the entire radiative heat of a typical 1K×1K long-wave Dewar, a typical 1K×1K medium-wave Dewar, and a typical 640×512 medium-wave Dewar when the window surface was facing a gray body with an emissivity of 0.9 was calculated. Among them, the calculated radiative heat of the 1K×1K long-wave Dewar was 176 mW, accounting for 52% of the thermal load; the calculated radiative heat of the 1K×1K medium-wave Dewar was 166 mW, accounting for 49% of the thermal load; and the calculated radiative heat of the 640×512 medium-wave Dewar was 74 mW, accounting for 37% of the thermal load.