Abstract: When the reflection optical system of periscope mirror rotates around mechanical shaft, the reflected image in the view field rotates around the optical axis which greatly affects observation. Offsetting image rotation prisms can eliminate the influence by spinning in the opposite direction. Though the analysis of two widely used offsetting image rotation systems, the criterion of the assembly process is raised, two typical methods for offsetting image rotation prisms are concluded, and the detection of offsetting image rotation prism fixed before and after the resolution in parallel light is put forward to ensure the imaging quality. The method is proved to be effective in reducing the difficulties in alignment of offsetting image rotation prism system and improving image quality. The windage between the optical axis of the offsetting image rotation prisms and system norm axis can be controlled less than 30.