A method is proposed to restrain mid-spatial-frequency errors (MSFE) by iterative small tool polishing, which is based on the research of restraint methods of MSFE, as well as the characteristics of polishing tools for high-accuracy aspheres. An asphere with obvious MSFE after fine grinding is prepared for pre-polishing in order to remove sub-surface damage as well as to control the figure accuracy. Then the aspheric surface is corrected to higher accuracy by bonnet polishing method, which is followed by the restraint of MSFE in the surface using self-made dual-flexible adaptive polishing tool. The restraint of MSFE is not finished until Zernike residual does not converge through CGH testing. An asphere with a diameter of 150 mm and a maximum departure of 0.344 mm from best fit sphere are successfully polished through 3 runs.The form is improved to achieves 4.5 nm RMS from 76 nm RMS and Zernike residual converged from 22.72 nm RMS to 3.46 nm RMS. Results show that the proposed method is useful to realize fast and valid restraint of MSFE for aspheres.