A novel algorithm and experiment using phase retrieval to test aspheric mirror without auxiliary optical elements has been developed. To test aspheric mirror without auxiliary optical elements, a new algorithm has been invented to overcome the calculation difficulty due to the significant departure of aspheric surface from a sphere. In this algorithm, the aspheric surface is fitted into several annular ellipse sub-apertures, and each sub-aperture aberration can be retrieved the same as ball mirror testing. The whole aspheric surface can be retrieved by stitching all the subapertures together. An experiment has been carried out to test the validity of this algorithm.