Firstly finite element method is adopted to analyse surface deformation of a single prism under gravity when it is in three typical positions,and Zernike polynomials are fitted for Φ425 mm axial deformations of the prism surfaces,corresponding wavefront error figures are plotted,a design of prism is choosed. Then Φ360 mm light beam is sampled and ray tracing for double prisms in several typical relative position,and corresponding total wavefront error of double prisms under gravity are computed.