Aiming at the vertical large plane interferometer, the method of threeflat test with Zernike polynomials was used to study the influence of the clamping and gravity deformation of the large plane on the absolute surface test results. The method of ANSYS finite element analysis was used to simulate the clamping and gravity deformation under different parameters. The clamping parameters of the ring width of 15 mm and the plane thickness of 90 mm are optimized, and the deformation quantity peakvalley value is 0.023λ (λ=632.8 nm). The result of simulation experiment and comparative analysis base on the threeflat test with reference plane deformation show that the clamping and gravity deformation of the large reference plane in the vertical large plane interferometer is not only effects on the reference surface shape of the reference plane itself, but also affects the absolute surface shape of the large plane without deformation, and the surface deviation peakvalley value is basically 0.011λ, so the clamping and gravity deformation influence can′t be ignored. This study will provide references for future research on deformation compensation.v