The X-ray radiation ionization processes at different altitudes of atmosphere with high-altitude nuclear explosions are numerically simulated, and the temporal and spatial distribution curves of electrons are shown in this paper. The electron density has one maximal point with 1 kt equivalent explosion at 80 kilometers, which is near the explosion point. It is different from the explosion point above 80 kilometers, and the electron density has two maximal points in that case. One is near the explosion point, and the other is near 90 km altitude, where the ionization effect range of high-altitude nuclear explosion is widest. The X-ray of high-altitude nuclear explosion has no effect at below 70 km altitude. With the increase of radiation angle, the altitude of the second electron density maximal point increases, and the cut-off altitude of X-ray increases too.