Aiming at the variational momentum of the zero position overshoot problem when rear mounted reticle with projected brightness was applied in a certain type of low-light-level (LLL) sight mirror, the zero stability mechanism of this kind of LLL sight mirror was analyzed. The impact test shows that the main reason for the variational momentum of the zero position overshoot is the rotation of the combined prism in the optical system during shooting. The overall size and mounting size tolerances of the combined prism were optimized, the gap between the two sizes was less than 0.02 mm, and the variational momentum of the zero position (Δd) of the LLL sight mirror was less than 0.26 mil, meeting the requirements of Δd≤0.7 mil specified in the system technical specifications. The key points and solutions of variational momentum of the zero position control in design and production of similar scope were given, which provided an effective analysis basis for improving the zero stability of the scope.