In order to always obtain the optimal images of parts in the assembly process, the problem of automatic optimization of microscopic vision illumination intensity was investigated. To realize the illumination of parts edges and surface detail, the illumination automatic control system based on coaxial and ring light source was developed. The illumination optimize performance of four types of image quality evaluation function was compared and analyzed, the image quality evaluation methods based on Roberts gradient sum function was proposed and applied in developed assembly systems. The results show that the process of illumination optimization is fast and accurate, the contrast of the target edge is significantly enhanced and the parts recognition error is eliminated after illumination optimization. Moreover, the precision automatic assembly requirements of miniature parts can be met.