To meet the requirement of on site testing in the manufacture of large mirror surfaces,a new method using phase retrieval technology was developed.A phase retrieval testing system based on the wave field at defocus was established together with the phase retrieval algorithm that serves the system.The testing system is simple and free from vibration,based on which,the error sources with testing precision and scope were analyzed in detail.The reliability for practical testing was also studied according to the requirement of on sit testing.The testing for a ball mirror of 430 mm aperture was carried out,where the affection from different testing configuration was also studied.The testing result showed high consistency compared with the result from interferometer.It shows that this method is feasible for high accuracy and suit for the requirement of polishing process.