Abstract: In this paper, it is assumed that at high latitudes when the air of the auroraloval is heated and then the molecule enriched air rises during the magnetic stormmain phase. This kind of molecule enriched air moves toward middle and low latitudes due to diffusion and interaction with the background neutral wind. Where itarrives, the electron loss coefficient increases and therefore the negative ionosphericstorm occurs. Based on this, a simulation is conducted and the relation between theonset times of the negative phase of ionospheric storms and the main phase of magnetic storms is given. The problem about the "time forbidden zone" of the occurrences of the ionospheric storm negative phase is also discussed. The results of calculation agree rather well with those of statistics.