Aiming at the time-consuming problem of traditional Monte Carlo calculation for radiation transmis? sion，this paper develops an improved Monte Carlo method，which solves the reflection and scattering energy through proportional iterative accumulation method，thereby greatly reducing the calculation time. The direct evalua? tion method is introduced，and the closed system containing participating media（square and circular as examples） is used as an example to analyze the influence of the grid density，the number of emitted energy beams and the physi? cal parameters on the calculation accuracy of the improved Monte Carlo method. When the optical thickness is 0.005，the relative root mean square error of radiation flux for the square cavity and the circular cavity is 0.002 5 and 0.002 3 respectively by using the improved Monte Carlo method，while the corresponding errors are 0.008 0 and 0.003 7 respectively when the traditional Monte Carlo method is used. It can be seen that under the same calculation conditions，the improved Monte Carlo method has higher accuracy for the radiation heat transfer problem. The influ? ence of the tracking energy beam number on the calculation error is further studied，and the fitting relationship be? tween the calculation error and the tracking energy beam number is given，which provides support for the selection of the energy beam number.