Abstract: Lateral shearing interferometry is an important optical measurement technology which obtains the wave-front phase distribution directly. Due to the ability of measuring surface shape of large deviation without high precision reference plane, it has been used in various fields;however, the selection of shearing displacement depends on work experience. In order to provide an explicit determination method, a shearing displacement determination method based on Zernike polynomial fitting is proposed, and the relation between shearing displacement and measurement error is given by computer simulation and experiment.