The positioning state of the imaging detector assembly has an important influence on the performance of the photoelectric reconnaissance system. The tilt, translation, rotation and position of the detector image plane will bring multi-dimensional errors during the installation and adjustment process, leading to the image plane imbalance of the imaging detector in the over-the-horizon observation system, and resulting in image blur and optical axis offset. A set of optical measurement system was designed and built, which adopted the image sharpness evaluation method of gray gradient to automatically judge the installation and adjustment errors of imaging detector. The installation accuracy could reach 0.015 mm, and the qualification judgment of the installation could be given according to the errors of the gray gradient value of the multi-region of interest (ROI) image of the optical measurement system. The optical precision assembly system lays a foundation for the realization of multi-dimensional accurate quantitative assembly of the detector by "mechanical agent".