Wepresentamethodthataccuratelymeasureslargeopticalsurfacesbeforepolishingusingalasertracker.Usingthescanningmodeofthelasertrackerconsiderablyimprovesmeasurementefficiencyandminimizesthedominanterrorscausedbyenvironmentalchange.WeusethismethodtomeasureaΦ1.3-masphericmirrorandobtainameasurementuncertaintyof0.72μm(rootmeansquare,RMS).