In order to study the scattering characteristics of weak defects on the surface of precision optical elements, a scattering theoretical model of weak defects on the optical surface is established based on vector scattering theory and bidirectional reflection distribution function. The variation of bidirectional reflection distribution function with scattering angle is simulated, and the effects of incident angle, incident light wavelength and defect size on the scattered light characteristics of defects are analyzed. Based on the analysis of simulation data, the influence of light source parameters on scattering characteristics is simulated and analyzed, which provides a theoretical reference for the selection of system parameters for precision surface defect detection using dark field imaging method. The range of incidence angle for defect detection is 30° ~ 50°. In the visible light range, the wavelength range of 380~500 nm is more conducive to defect detection. In addition, by studying the variation law of scattering field when the defect size changes, it provides a reference basis for distinguishing the shape and size of defects.