A null phase measuring deflectometry (PMD) is presented to measure the three-dimensional shape of aspheric mirrors with radial symmetry. Curved fringes displayed on a mobile screen are reflected by the surface under test, and a camera captures the fringe patterns. By analyzing the captured fringe patterns, the deviations along the normal of the mirror’s surface can be restored by integrating the slopes of these deviations, and then the surface under test can be reconstructed. Compared with non-null PMD, the qualitative information of surface in null PMD can be shown fast and easily for a surface with large deviations in the grinding stage. Computer simulation and experiment are presented to validate the presented method.