In order to study the influence of optical surface residual on the optical image system, a kind of method dividing spatial frequency based on the exit pupil phase of the optical system is proposed and the analytical expression of modulate transfer function which is influenced by mid-spatial frequency (MSF) errors is provided. The feature spatial resolution of the image whose modulate transfer function (MTF) is influenced by MSF is obtained. By comparing the influence of low-spatial frequency (LSF) errors and MSF errors, it is concluded that there is a sudden drop of MTF on the point of feature frequency under the influence of MSF.