This paper has proposed ultra-lightweight design and optimization for a primary mirror of a space camera with Φ210 mm aperture to improve surface quality and reduce its launching cost. Three-point support on mirror back is selected as the support method and the locations of the supports are carefully studied. Through the topology optimization, the mass distribution of the back of the mirror and the distribution of the removable part are obtained. The initial structure of mirror is light weighted on its back applying triangular hole light-weighting method. Multiple-object optimization which combines mass and surface figure error along X-axis(detection direction) as optimization object under constraint of surface figure error along Z-axis(machine direction) is carried out on the initial structure of mirror. Finally, a mirror with RMS(Root Mean Square) as lowas 0.18 m with X-axis straight down and 2.38 mm with Z-axis straight down is achieved. The mirror weights 0.568 kg, surface density of it is as low as 16.9 kg/m2. And the primary frequency of X,Y,Z are more than 500 Hz. Through the FEM analysis and vibration test, verify the good mechanical properties of the mirror structure. The results show that the optimal design method of this paper is reasonable for the lightweight mirror.