In order to realize the figure accuracy of λ/100 peak-vally (PV) value for the lithographic projection objective, the effect of gravity deformation on the optical performance of large-aperture Fizeau interferometer with super accuracy has been deeply analyzed. The optical configuration designed with a wavefront aberration of λ/1000(PV) and a numerical aperture(NA) of 0.36 in image space are employed to test the spherical surface with a diameter larger than 300 mm. One element in the configuration is analyzed by the finite element method (FEM) under two supporting types: adhesive support and retainer support. Based on the finite element analysis(FEA) results, surface deformation and body displacement due to gravity are obtained under a certain condition of load. By Zernike polynomials, the surface deformation is fitted, the wavefront error is acquired, the optomechanical conversion is finished, and a proper supporting type is proposed. When this supporting type is applied to the whole configuration, the wavefront aberration of the system achieved is better than λ/100 (PV). The result verifies that super accuracy can be reached. Besides, the data offered here will be useful for a reasonable interferometer design.