A novel three-dimensional (3D) shape measuring method based on phase measuring deflectometry (PMD) is presented, which can be used to measure the aspheric mirror during the stages of fine grinding and the beginning of polish. The proposed method measures the absolute height and slope of the specular surface with incidence ray, camera pinhole and dummy assistant surfaces. During the measurement, a camera with an external stop is used to realize pinhole camera model. The external stop represents the position of the camera and the incidence ray corresponding to the pixels of the camera can be obtained by moving LCD screen once. The procedure of the test is simple and the result is believable because it has no restrict requirements for the experimental devices and no need of other auxiliary devices. What′s more, this method measures the specular surface with dummy auxiliary surfaces instead of the intersection of the incidence ray and the camera ray. So there is no need to calibrate the camera ray. This method can be used to test the specular surface with high pre ? cision because the calibration error has little effect on the result of the testing. Computer simulation and experi? ment validate the feasibility of the proposed method.