Usually, a lens is used in a large-aperture mirror testing system. The deformation caused by gravity of the lens seriously influences imaging quality, so it is a critical work to reduce deformation of a support system by optimization. We investigated a large-aperture lens with 1.5 meter diameter in a testing system, and the 3D solid model constructed by Solidworks was imported into Ansys software for finite element analysis. The position of support points and support force were optimized with Design-Expert software and OPT workbench of Ansys. The front and rear surfaces of the distorted mirror were simulated with Zernike polynomial. The root mean square (RMS) values of the two surfaces′ deformation are 1.85 nm and 3.28 nm, Peak to valley (PV) values are 9.4 nm and 24.4 nm. Meanwhile the RMS wavefront error is 0.998 nm, which meets the technical requirements.