Facing the flexible support problem of the ultra-light mirror for micro-remote sensing payloads, a flexible support structure with folded-arm beam was proposed. According to the structural characteristics of the mirror, a parametric model of the flexible support structure is established. The second-generation non-dominated sorting genetic algorithm is used to establish a multi-constrained and multi-objective parameter optimization model. The key parameters of the flexible structure are optimized. The minimum size is only 1.30 mm. The strength of the bonded area is checked and the safety margin is 3.6. Finally, the engineering analysis and mechanical test of the mirror assembly are carried out. The flexible structure designed can ensure that the surface accuracy of the ultra-light mirror is better than 3 nm, when it is under the gravity and temperature is changed by 4℃. The relative error of simulation analysis and mechanical test is less than 10%. Finally, the mirror surface accuracy of the mirror assembly was detected. Before and after the mechanical vibration test, the surface error RMS of the mirror is 0.020λ, indicating that the mirror assembly has good stability. The feasibility of the design method and reliability of the flexible support structure with folded-arm beam proposed in this paper are verified.