Aiming at the problem that machine vision is difficult to detect defects on reflective surfaces, a defect detection method based on phase measuring deflectometry is proposed in this paper. This method uses an LCD screen as a structured light source, Gray code pattern combined with phase shift pattern as structured light, which is projected onto the measured surface through the LCD screen, and monocular camera is used to capture the reflected pattern of the structured light. Firstly, the image captured by the camera is decoded pixel by pixel to obtain the wrapped phase and fringe order. Secondly, the absolute phase image is generated by unwrapping phase and eliminating the error. Thirdly, Canny operator is used to detect the edge of the image to segment the projected region and the unprojected region. Finally, Canny operator is used to detect the edge of defect in the projected region to realize the detection and location of surface defect. The experimental results show that under the condition of low and constant ambient brightness, phase measuring deflectometry can efficiently detect the defects with deformation characteristics on the reflecting surface, with an accuracy of 0.1 mm. It basically meets the requirements of high precision, high efficiency and low cost detection for various defects on the reflected surface.