How to further improve the lightweight ratio of high-performance meter-level space mirrors is one of the core issues in the field of large aperture optomechanical structure design. In this paper, a primary mirror with a clear aperture of Φ1200 mm was developed for a high-resolution space camera, which achieves the goal of designing area density below 40 kg/m². The SiC mirror blank was prepared by gel injection molding and reaction sintering process. The state of the optical axis being horizontal was taken as the testing state to simplify the supporting structure. Novel lightweight measures such as an alternate arrangement of main and auxiliary stiffeners and the addition of lightweight holes on vertical walls were used inside the semi-closed mirror blank. The distributed datums were used to replace traditional datum settings, which reduces the machining area of datums by more than 80% and improves machining efficiency. Through parametric modeling and integrated optimization, the optimal structural parameter combination of the mirror blank was determined, with the final design weight of 46.9 kg. The RMS value of self-weight deformation of the mirror blank is only 2.87 nm under the state of the optical axis being horizontal, and its free fundamental frequency is 602 Hz, indicating that the primary mirror proposed in this paper has good dynamic and static characteristics. After machining, the measured weight of the mirror blank is 51.3 kg, about 9.4% overweight, and the facesheet is about 1-mm thicker than the design. At present, the mirror has already been polished to RMS λ/8 (λ=632.8 nm) of surface shape accuracy, with no obvious print-through effect observed.