The aspheric normal intersects the optical axis at different points with different angles and the surface can be expressed by the normal. The method based on phase measuring deflectometry (PMD) for testing the surface normal is presented. The aspheric surface under test shifts accurately along the optical axis relative to the measurement system. The camera observes the patterns reflected off the testing mirror at every scanning position while scanning the testing aspheric surface along the optical axis. Through the phase distributions, the relation between the intersection point and the angle of the surface normal with the optical axis can be gotten. In this method, the surface can also be expressed in the orthogonal coordinate.