A horizontal placing method for the lens of high-precision interferometer is given according to its operating mode. Using Ansys Workbench finite element software, simulation and analysis are carried out on the deformation of mirror surface which is caused by gravity and the support structure. Based on the Finite Element Analysis (FEA) for opto-mechanical structure, optimize the mechanical design of the lens repeatedly, so that the systematic errors caused by support structure get to minimization. Fit deformed mirror surface with Zernike polynomial as an interface tool, and evaluate the effect of support structure on optical performance. The analysis results show that the RMS of aberration of the interferometer lens is less than 1/50λ which can be fulfilled with the image quality demand under the support method.