The large-aperture off-axis three-mirror optical system, with its advantages of multiple optimization variables, simple structure, wide field of view, unobstructed view, no chromatic aberration, and high resolution, is widely used in space infrared cameras. Before entering orbit, space infrared cameras must endure severe mechanical environments during the launch phase. After entering orbit, they are subjected to microgravity and complex space thermal conditions. Under a series of external loads, the ideal focal plane position of the camera is likely to shift, leading to defocus issues in the infrared camera and ultimately failing to achieve the mission objectives. Therefore, improving the load adaptability of large-aperture off-axis three-mirror infrared cameras and reducing the risk of defocus in orbit have become key research areas. This paper focuses on the secondary mirror of the off-axis three-mirror optical system as the target for focus adjustment, and carries out adaptive design and experimental verification for the application scenarios of the focus-adjusting secondary mirror. Starting from the weight and positional tolerance accuracy requirements of the optical elements in a large-aperture off-axis three-mirror infrared camera, and considering the feasibility of the corresponding focusing mechanism, a focusing scheme using the secondary mirror as the focusing object was determined. Based on the constraints of the focusing secondary mirror's application scenarios, adaptive design was performed for the secondary mirror and its support structure. A scheme utilizing silicon carbide for the secondary mirror material and central support was adopted to achieve a lightweight and compact structure for the focusing secondary mirror. A parameterized model (Fig.1), a finite element model (Fig.2), and an optimization function for the focusing secondary mirror were established. Structural optimization of the secondary mirror was carried out based on the parameterized model, with surface shape accuracy and weight as optimization constraints. To verify the feasibility of the design, machining, assembly, and experimental validation of the secondary mirror and its support structure were conducted.Based on the parameterized model and optimization function, the structural optimization of the secondary mirror was completed (Tab.3), and the simulation analysis results met the design requirements (Tab.4). To verify the engineering feasibility of the focusing secondary mirror scheme, optical machining of the mirror surface and secondary mirror structure was completed (Fig.3-Fig.4). Assembly and testing of the secondary mirror and its support structure were carried out, and no change was observed in the mirror surface shape accuracy (Fig.5). After assembly, the mirror was installed on the focusing mechanism and underwent vibration testing (Fig.6). The RMS change in mirror surface shape accuracy after vibration testing was 0.001λ (λ=632 nm) (Fig.7). The variation in mirror surface shape accuracy may be due to the redistribution of bonding stresses between the secondary mirror and its support during the vibration process. However, the final surface shape accuracy of the focusing secondary mirror meets the optical system's requirements.To improve the imaging reliability of the off-axis three-mirror infrared optical camera under harsh mechanical and thermal conditions, a focusing method using the secondary mirror as the focusing object was proposed, based on the feasibility of implementing various focusing objects. Additionally, to reduce the load on the focusing mechanism and lower the mechanical response, a compact and lightweight method for central support of the secondary mirror was proposed. Through component assembly and mechanical testing, the secondary mirror maintained good surface shape accuracy, validating the engineering feasibility and reliability of the design. This provides a reference solution for the focusing method and support design of the focusing secondary mirror in off-axis three-mirror infrared cameras.