To control the mid-spatial frequency error of a flat optics by Magnetorheological Finishing(MRF), the correction ability of frequency error by the MRF removal function and the suppression method of residual error created by the MRF were investigated. Firstly, the changes of mid-spatial frequency PSD1 (Power Spectral Density 1)errors of the optics and the PSD curves before and after simulating the MRF were compared and the correctable frequency error range of the MRF removal function was analyzed. Then the effects of MRF processing parameters such as removal depth, tool-path intervals and the size of removal function on mid-spatial frequency PSD2 were analyzed based on uniform remove way and a method to suppress the mid-spatial frequency PSD2 error was established. Finally, the flat optics with a size of 400 mm×400 mm was polished by the MRF. Experimental results indicate that the wavefront PV of the flat optics is converged from 0.6 λ to 0.1 λ, the PSD1 error is converged from 5.57 nm to 1.36 nm, and the PSD2 error changes from 0.95 nm to 0.88 nm. It is concluded that the MRF has the abilities to control mid-spatial frequency error of large aperture flat optics by optimizing and choosing MRF processing parameters and corresponding strategies.