Abstract: Dispersive electron gun with large incidence face is one of the key components in a large magnetospheric substorm environment simulation system. In this paper, the deflection effect of geomagnetism field on ground low-energy electron irradiation is studied analytically. The deflection distance and angle are shown to depend on the geomagnetism field and the accelerate voltage. The calculation results are validated by experiments, which may serve as a guidance for the development of dispersive electron gun with large incident face in magnetospheric substorm simulations.