A method based on Ronchi test and synchronous phase detection technology is presented to measure the aspheric mirror, where the point source is located beside the axis of the surface. A liquid crystal display screen displays the vertical and horizontal sinusoidal gratings and a camera captures the deformed fringe patterns generated by the reflection of the mirror. By four-step phase-shifting technique, the phase distributions are obtained. According to points with the same phase on the deformed fringes and grating, the transverse aberration of all tested points can be calculated, while that of the relevant ideal mirror can be worked out by geometry algorithms. Then the residual transverse aberration can be taken and thus the gradient of shape deviation can be determined. Tested mirror can be reconstructed after restoring shape deviations by integral processing. In experiment, the computer-generated fringe will bring a no-error phase shift and control the angle of two gratings to be 90°. A marked point is used to guide the phase unwrapping procedure. Computer simulations and preliminary experiment validate the feasibility of this method.