In this paper, the Monte Carlo method is used to simulate atmospheric ionization effects of the prompt radiation from high altitude nuclear explosion (HANE). Simulation gives the instantaneous spatial distributions of the electron density in atmosphere. The atmosphere is a kind of non-uniform media, whose density drops rapidly with the increase of height. Therefore, a method, which samples the mass distance to replace the step length of the particle, is used to considerably increase the computing speed. The simulation results show that there is an obvious difference between additional ionization regions of different height of nuclear explosion. As the explosion-height increases, influence range of the prompt nuclear radiation will be wider, but the electron density will be lower.