With the development of remote sensing technology, new camera requires large aperture variable-direction mirrors with higher accuracy. A new type of back support structure with highly stable support design was proposed for 1 000 mm×700 mm large-aperture variable-direction mirror. Compared with the conventional design, it had the characteristics of small volume envelope, light weight and wide adaptability. The mirror was made of ULE, and the light weight form of honeycomb layer was used. The mirror was mounted by means of a 3-point ball joint flexures system on the back. The stress in reflector assembling and the influence of thermal stress on the orbit was eliminated. It reached the stress-free support of the mirror. The problem of consistency between the ground test and in orbit was solved through the design of gravity unloading structure and the optimization of the unloading force. The simulation analysis shows that the surface RMS is 0.006λ(λ=632.8 nm) at 90° mirror testing direction. The surface RMS is 0.005λ at 75° camera imaging testing direction Ⅰ. The surface RMS is 0.011λ at 45° camera imaging testing direction Ⅱ. The first order modal of the component is 83.2 Hz, which has a high design stiffness and can meet the requirements of the mechanical environment during launch. This large-aperture variable-direction mirror design can meet the needs of new types of remote sensor and can provide a reference for the design of similar mirrors.