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 according to the gray body assumption and compared with the measured values. The theoretical calculated value and measured value of the cold side radiation heat of experimental dewar 1 were 155 mW and 136 mW， respectively， with an error of 19 mW； the theoretical calculated value and measured value of cold side radiation heat of experimental dewar 2 were 87 mW and 79 mW， respectively， with an error of 8 mW. After initially testing the accuracy of the calculation program， the cold side radiative heat of an engineering typical 1K×1K long-wave dewar when the emissivity of the window facing it was 0.9 was calculated and measured experimentally. The theoretical calculated value was 127 mW， and the measured value was 110 mW， with an error of 17 mW.