Abstract: With the rapid development of infrared technology, the applications of infrared continuous zoom lenses are continuously expanding. The mechanical structure is the core carrier that ensures the precise pose of the lens elements for zoom lenses. The accuracy of the lens eccentricity, tilt, and axial spacing is a key factor that affects the imaging quality of the system. To satisfy the imaging performance requirements of an infrared continuous zoom optical system, the mechanical structure of a lens was designed according to the precision indices of an optical system. Precision error simulation analysis was used to accurately allocate the machining and assembly tolerances of the key components. With the goal of improving the axial spacing lens assembly error, a closed-dimension chain model was established to solve the closed-loop deviation, and a gasket grinding compensation process was adopted to regulate the axial assembly accuracy. A precision assembly process suitable for a zoomed-in system was designed. The experimental results showed that the proposed structural design and assembly scheme could effectively control the lens-pose accuracy and satisfy the imaging requirements of the system, demonstrating good engineering applicability and reliability.