In this paper, combined with the principle of phase measuring deflectometry, the problem of defect detection is converted to the problem of extracting abrupt areas using absolute phase difference to solve the problem that traditional machine vision is difficult to detect surface defects of mirror-like surface. First, the coordinates and dimensions of the surface to be measured are obtained by gray projection method, and the area of the surface to be measured is segmented. Then, phase extraction and phase unwrapping are performed on the measured surface and the absolute phase distribution map is used as a difference with the absolute phase distribution of the reference surface to obtain an absolute phase difference map. Finally, the phase difference map is processed in frequency domain to filter the periodic components, and the Sobel filter is used to extract the phase difference map's abrupt areas to obtain surface defects. The experimental results show that the defect detection method based on phase measuring deflectometry can detect surface defects of mirror-like objects with a 0.180 mm detection accuracy.