Abstract: Mid-spatial frequency(MSF) wavefront error leads to laser damage during the use of optics. Therefore the accurate evaluation and modification are necessary in manufacture. A unique way based on multi-sample analysis was proposed for error frequency. Several outstanding frequencies were found in every sampling direction. After eliminating with the help of these frequencies, along with certain sample direction, unqualified frequency errors in 0.041 7 mm-1, 0.085 8 mm-1,0.041 7 mm-1 were down to 23.9%, 18.3%, 29.2%,respectively, while the MSF in whole optic surface decreased to half. According to the method above, the uncertainty caused by directionality and regionality of MSF in optic surface can be restrained.