In order to reduce the influence of the mass and sizes of mirrors on the quality of space camera, such as optics quality, mechanics quality and stability of thermal dimension of structure, the necessity of lightweight for mirrors was introduced. Material selection, substratum′s back shape, lightweight pattern and support pattern were taken into account in the design of large-aperture mirror in space camera. A new design with open bottom, triangular aperture was proposed based on theoretical and finite element analysis. Through the rational design of support structure parameters, the surface accuracy of mirror meets the needs of the imaging quality (PV≤63.2 nm) under environment of gravity.