A Φ1550 mm aperture space mirror’s surface figure RMS was required to be superior to 1/50λ (λ=632.8 nm) under the zero-gravity orbit environment. In order to simulate the state of weightlessness and reduce the influence of gravity in the mirror’s surface figure test with horizontal optic axis, the mirror was actively supported by multiple forces to unload the gravity and the forces’ parameters were optimized. Firstly, the principle to determine the value, the number of support points and the initial axial position of each unload force was proposed based on dividing the mirror into blocks. Secondly, with the optimization goal of the mirror’s surface figure RMS be superior to 0.002λ under the function of gravity along with all unload forces, a structural FEM model was established. Taking the positions of all unload forces along the optic axis as optimal variables, influences on target were analyzed and quick optimization points were concluded to simplify the optimization. Finally, the mirror’s surface figure RMS when unloaded was found minimal of 0.00145λ. Putting the parameters of the optimization result into use of the surface figure test of the mirror with horizontal optic axis, it turned out that when the mirror revolved around the optic axis 0°, 120° and 240°, the surface figure RMS were 0.0157λ, 0.0161λ and 0.0159λ respectively and the figures were consistent, which proved that the gravity impact was eliminated effectively. The optimization method for gravity unload is flexible and efficient which guarantee the large-aperture mirror’s high-precision machining and space mission.