The deformation of large- aperture optical mirror is induced by gravity. It is very important to analyze the support structure and format in grounding and testing. By finite element software, a Φ620mm mirror is created and its theoretical analysis about three comparative testing programs of vertical support is proposed. Zernike polynomials are used to fit optical surface and separate corresponding aberrations. By reducing the aberrations with great impact on mirror, at the same time, RMS excel 0.025λ (λ=632.8nm) has been achieved. Experiments of three kinds of support verify that the entirely bound by strip is the best way in all supports. It affords a more scientific and rational support to large-diameter optical mirror reflection. Through research and analysis of support on large-diameter mirror by finite element, it provides a theory of design and guidance for the development and test of reflection mirror in future.