The requirement of the large aspect-ratio rectangular plane mirror for a certain space remote sensor is that its weight should be reduced as much as possible and its surface deformation error (RMS) should be less than λ/50 (λ=632.8 nm) at the operation temperature of 20±5℃. The material selection of the mirror and its supporting structure is presented. The weight reduction and supporting modes of the mirror are analyzed. According to the shape features of the mirror, the local width of the back of the mirror is increased and the backside of the mirror is designed to have three supporting points. Through finite element analysis, the mirror and its flexible supporting structure are optimized and determined. The analysis results show that the displacement and surface deformation of the mirror meet the design requirements. Finally, the dynamic characteristics of a simulated mirror assembly is tested in a mechanical environment experiment. It is shown that the structure of the mirror meets the design requirements.