Applying phase retrieval to mirror metrology can simplify the measurement system and increase the ability of resisting the vibration effects. The capability of phase retrieval testing system for mirrors is described from two aspects: its hardware and algorithm. According to the theory of Fourier optics and geometry optics, the pixel size and the total size of CCD spherically limit the f-number and the departure of a wavefront respectively. The adjustability of phase retrieval algorithm to different figure errors is observed afterward. A series of simulation experiments show that the algorithm is high-powered when the period of figure errors is about 31.4mm, and the performance is weaker when over or under that frequency; but the algorithm is reliable when the RMS value is up to six wavelength or as small as one percent wavelength.