The influences of mid-spatial frequency surface errors on the illumination field uniformity and the penumbra width of off-axis illumination are analyzed herein. The analytical relationship between peak-valley (PV) values of the errors and the coherence factor on the line spread function (LSF) distribution under the dipole illumination is derived, and the effect of the mid-spatial frequency error on the LSF of the relay lens group of the photolithography illumination is numerically analyzed. Mid-spatial error describes the reduced uniformity of the illumination field and the increased penumbra width. Using actual manufactured surface errors in conjunction with commercial optical design software to simulate the relay lens group in the photolithography illumination, theoretical accuracy is verified. Therefore, this simulation method can be used at the design stage using the actual manufactured profile, which can provide a reference for optical designs.