In order to solve the difficulties in testing surface of long focal length mirror using interferometer,a novel optical metrology using phase retrieval is developed. In this method, tested mirrors is illuminated by a coherent point light source, a series of diffraction intensity patterns near focus are collected, then phase retrieval algorithm reconstructs the surface error of tested mirror. Compared with interferometer, it simplifies the metrology setup and requires no special environment. Mathematic model of phase retrieval measurement is found according to diffraction theory, and is solved by iterative algorithm which derives from Gerchberg-Saxton algorithm. Validity of this method is tested by simulations and an experiment. A spherical mirror with curvature radius of 8 700 mm and diameter of 145 mm is tested using phase retrieval. The position of intensity patterns are corrected and appropriate patterns are chosen to be computed. The reconstructed result is correlated well with that tested by simultaneous phase-measurement interferometer.