Support structure is one of the most important factor of high- precision optical components. The relationship between the gravity deformation and caliber and thickness under three-point support is studied. The combination of numerical analysis, finite element method (FEM) and experimental validation is used to complete support under its own gravity deformation of three-point support. Firstly, using sheet theoretical to derivate the relationship between the gravity deformation and caliber, thickness under three-point support; and then, using the FEM analysis of the gravity deformation plane mirror size in a range of different thicknesses and calibers. Finally, the correctness of the model is validated by experiment. Under the basic of the sheet theoretical, the relationship between the gravity deformation and material, caliber, thickness and form is established completely, and it provides theoretical basis for quickly work out the gravity deformation under three-point support.