Aluminum mirrors are widely used in aerospace and space remote sensing satellites. In order to improve the lightweight rate of aluminum mirrors and reduce the launch cost of commercial remote sensing satellites, a lightweight aluminum mirror with open back, triangular hole structure and three-point support was designed. Based on this structure, firstly, with the minimum mass as the optimization objective, the thickness and layout of the stiffener on the back of the mirror as the optimization object, and the minimum flexibility as the response constraint, the finite element method was used to optimize the size of the thickness of the stiffener on the back of the mirror and the topology optimization of the layout of the stiffener, and the lightweight rate of the aluminum mirror exceeded 63%. Then, the fixture was designed to carry out the single point diamond turning test. Finally, the surface shape accuracy of the mirror was measured by laser interferometer. The results show that the surface shape value of the mirror is PV(peak to valley) 1.48 μm, which can meet the requirements of commercial remote sensing satellites.