The thermal load of the cryogenic infrared detector Dewar is a comprehensive indicator characterizing the adiabatic capacity of the Dewar. Radiation heat is a part of the thermal load. When calculating the radiation 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 radiation heat transfer between surfaces where emissivity， transmittance， and reflectance vary with wavelength. To enhance the calculation accuracy of the Dewar''s radiation heat， based on Monte Carlo principle， a 3D Studio Max model was employed， model information was extracted， and a program was developed， resulting in a set of general calculation program for the Dewar''s radiation heat based on the radiation transfer factor. To preliminarily verify the accuracy of the calculation program， the cold side radiation 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 the 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 the error of 8 mW. After initially testing the accuracy of the calculation program， the cold side radiation heat of an engineering typical 1 K×1 K long-wave Dewar was calculated and measured experimentally when the emissivity of the window facing it was 0.9. The theoretical calculated value was 127 mW， and the measured value was 110 mW， with the error of 17 mW.