If the technology is not properly controlled in the machining process of precision optical elements, although the distribution range of defects such as scratch and pitting is small, it has great influence on the performance of optical system and is very destructive. At present, the surface defect detectors are mainly used for off-line detection of planar or spherical optical elements. The optical machine tool was used as the motion platform, the method of scattering imaging was adopted in dark field, the uniform illumination system with multi-beam was designed, the recognition algorithms of fine features on surface defects were studied, and in-situ detection and evaluation of surface defects for large-aperture optical elements were achieved. Calibration results show that the width deviation of surface defect is 2.05% and the length deviation is 2.39%, which meet the index requirement. On this basis, automated in situdetection is carried out for Φ280 mm planar silicon mirror. The statistical data of different defects is given, and the problems of long non-machining time and locating error caused by multiple clamping in off-line detectionare solved.