Abstract: In the field of high-precision optical elements, especially the planar optical elements with large aperture, the deformation by gravity becomes an inevitable problem with the improvement of requirements for higher measuring accuracy and larger aperture. The system design of vertical Fizeau interferometer and the deformation theory of multi-point adjustable support method were introduced.By finite element method (FEM) analysis, the reasonable support structure is optimized and derived. Calibration of deformation of flat by gravity was obtained on 300-mm-diameter vertical Fizeau interferometer with liquid surface datum. Comparing it with detection result by three-flat method, the residual between two methods can reach 10.20 nm for peak valley (PV), and 1.56 nm for root mean square (RMS).