A calibration method of shear amount based on the optical layout of point source microscope (PSM) for lateral shearing interferometric (LSI) wavefront sensor is proposed. A simulation model of quadriwave LSI is introduced to analyze the influence of the window size of the spatial filter and the shear amount on the accuracy of the shearing wavefront feature extraction (SWFE) method. Simulation results show that the accuracy of the SWFE method deteriorates at small shear amounts. The proposed calibration method makes use of the optical layout of PSM to generate spot images of the point source at the detector plane of the sensor itself. The shear amount is calibrated by the relationship between the lateral distance of the spot images and the distribution of the diffraction orders of the grating. In the experiments, an SID4 wavefront sensor and a circular-aperture modified Hartmann mask LSI wavefront sensor are calibrated by the proposed method. A phase plate etched with patterns has been manufactured as the test specimen. The etching depth is characterized by the two wavefront sensors, and the testing results are compared with that by a commercial ZYGO^® interferometer. The feasibility and accuracy of the proposed calibration method are validated. This calibration method provides an easily conducted approach for calibrating the shear amount of LSI wavefront sensor with high accuracy.