Abstract: In order to realize on-line automatic detection of surface form deviation of spherical optical elements based on the existing optical detection system, the steps of image processing of interferogram were introduced and the method of fitting the wavefront of interferograms was mainly studied.Firstly the introduction of interferogram pretreatment process was presented. According to the optical elements which were mostly circular, the boundary circle of interferogram was fitted by the least-square method, so as to determine the circle center and raduis accurately. Then the algorithm of wavefront fitting with multi-quadric(MQ) interpolating function was analyzed.After the wavefront of optical elements was reconstructed, the values of EPV and ERMS were calculated. Finally the comparative analysis of the processing results of national standard interferogram was presented.Experimental results indicate that the wavefront fitting accuracy can be improved through boundary processing,the wavefront can be reconstructed ideally by fitting with MQ interpolation method,moreover, the measurement error can be controlled in 0.2 range for national standard interferogram, and in 0.03 rang for Zygo interferometer result, which can satisfy the requirements of the optical element on-line measuring precision.