Near-ground nuclear explosion fallout consists of radioactive particles of different particle size, and its motion has a large spatial and temporal scales. For the problem how to simulate the atmospheric γ ionizing radiation environment of the near-ground explosion fallout, in this paper, firstly, the mechanism analysis of atmospheric gamma radiation is carried out, the no-wind conditions are set up, and the theoretical model of fallout gamma radiation is established. Secondly, the corresponding numerical difference and integration algorithms are introduced and proposed. Finally, the simulation example of the radioactivity and radiative dose rate in the atmosphere of the 1000 kt Nevada near-ground explosion is given, a certain summary of the temporal and spatial evolution patterns of the radiation environment and the comparison of the results are accomplished, and the comparison reveals that the present model is able to calculate the theoretical maximum of the atmospheric radiation dose rate while ensuring the consistency of the activity results.