Large aperture rectangular mirror is one of the core components in large off-axis TMA camera, focusing on the primary mirror with clear aperture of 1250 mm×460 mm in Jilin-1KF01C satellite, the structural design method for 1.2 m scale mirror assembly with large aspect ratio was systematically discussed, and the developed mirror was verified in detail. Based on material properties and existing processes, the mirror body was made of reaction bonded silicon carbide, semi-closed lightweight form was adopted, the optimal combination of structural parameters was determined through two-objective global optimization. The final design weight of mirror body was 41.8 kg, with the facepanel 5 mm thick and the thinnest stiffener 3 mm thick. The classical back three-point support scheme was adopted, the structural parameters of the biaxial flexure hinge in the flexible support were optimized, so as to take into account fundamental frequency and thermal stability of the component, and match the centroid position of the mirror body. The assembling process for the primary mirror assembly and the corresponding stress relief measures were put forward. Test results showed that the full aperture surface accuracy of primary mirror under testing gravitational condition was 0.016λ (λ=632.8 nm) in root mean square value, and the full aperture was 0.019λ after turning 180°. The tested 1st fundamental frequency of the assembly was 128.5 Hz, and the root mean square value of primary mirror basically remained stable after large scale random vibrations and wide range temperature cycles. The primary mirror assembly not only has good dynamic and static characteristics, but also has the features of high surface accuracy and good stability, which can meet the application requirements of high-performance space optical system.