Mid-spatial frequency surface errors (MSFSE) can cause small angle scattering in optical systems. In order to determine a reasonable tolerance for MSFSE in optical design and processing, this paper conducts a quantitative study on the impact of MSFSE on the modulation transfer function (MTF) of optical systems. Under diffraction-limited conditions, we derived an expression for the influence of MSFSE on the MTF of optical systems and analyzed it. Then, we verified the theoretical derivation results through optical design software simulation. Assuming that the optical system has a sinusoidal MSFSE on the pupil, we performed the Fourier transform on the pupil function and squared it to obtain the point spread function (PSF), and then performed the Fourier transform on the PSF to obtain the optical transfer function (OTF) of the optical system. By taking the OTF mode, the expression of MTF under the influence of MSFSE can be obtained. By comparing this expression of MTF with the MTF of an optical system without MSFSE under diffraction-limited conditions, the quantitative impact of MSFSE on the MTF of the optical system can be obtained. Theoretical calculation results indicate that sinusoidal MSFSE can lead to different losses of MTF at different spatial frequencies and that the changes in MTF losses are periodic. The maximum loss ratios of MTF in optical systems caused by sinusoidal MSFSE with peak-to-valley (PV) of 0.030 μm, 0.095 μm, 0.159 μm and 0.223 μm are 0.89 %, 8.80 %, 23.48 % and 43.31 %, respectively. The loss of MTF will increase nonlinearly with the increase of PV of MSFSE. The theoretical calculation results are consistent with the software simulation results.