When fabricating ultra-large aperture mirrors, it′s always very crucial to reduce the control difficulty and to lower the cost of the support system while providing the support accuracy required for on-line fabrication and testing. For this purpose, layout optimization method of supporting points for the equal-force support system is proposed. Firstly through surface fitting, surface figure accuracy of supported mirror is analyzed, and the objectives for layout optimization are auto-updated. Afterward support layout optimization, whose model has the capability of adaptive finite element analysis, of arbitrary-shaped thin flat mirrors is carried out. Sequentially load transfer structure for lightweight mirrors, which are widely used, is designed and the support location within each transfer is optimized. Finally the optimization method is applied to 30 m telescope (TMT) tertiary mirror and a 2-m aperture mirror. Results show that the proposed method has the required accuracy for the optical fabrication of ultra-large mirrors.