A kind of non-contact online spherical interferometer has been designed and developed with Twyman-Green interferometry principle in the laboratory. The CCD camera lens in this interferometer can produce relatively high resolution image of the interence fringe. This paper mainly focuses on the basic method for using MATLAB image processing software to digitize the interference fringe, and through the pretreatment of this interference fringe and the extraction of the skeleton, finally the wavefont function of the detected spherical surface was fitted by adopting the Zernike polynomial;through the MATLAB programming, the automated processing of the detection signal of this spherical interferometer was realised.