In order to explore the relation between the modulation transfer function (MTF) and the halo effect for low-light-level (LLL) image intensifiers, the MTF for the preproximity focusing electron-optical system is calculated according to the electron distribution on the microchannel plate input face. During the calculation, the halo effect from secondary scattered electrons is not treated. By tracing the trajectory of photoelectrons emitted from one point on a GaAs photocathode into the preproximity focusing electron-optical system, the electron distribution is calculated, namely the point spread function. The MTF for the preproximity focusing electron-optical system is numerically calculated according to the electron distribution, which is fitted. The results show that the fitting curve of the MTF is in agreement with the analytic expressions. When the spatial frequency is less than 50 lp/mm, the relative error is below 5%. This research provides theoretical support for further development of LLL night-vision technology.