The influence of mid-spatial frequency surface errors on flare is analysed. Power spectral density function is used to describe the mid-spatial frequency surface errors and analyze the flare. The concepts of spatial frequency and equivalent pupil are used to analyze tolerance. Based on the method, the lens with numerical aperture of 0.75 and working wavelength of 193 nm is analyzed under the requirement of the local flare intensity ratio is less than 0.5% in the scattering range of 2~10 μm. The spatial frequency range of equivalent pupil is 16~78. And it demands that the coefficient of power spectral density is 0.06 and the coefficient of power spectral density is 1.5. The results show that the way can be used to analyze the flare and tolerance the mid-spatial frequency surface errors in lithographic lens.