Adjustable optic mount is commonly used in the structure design of high precision optical system. Surface shape of optical element influenced by the supporting force adjustment is researched. Mechanical model of the adjustable mount is established, deformation of optical element is calculated with finite element method, and the surface shape is fitted with Zernike polynomials. The analysis results in different working conditions show that the root mean square (RMS) of surface shape is minimized when optical element is under uniform support; when supporting force changes, it causes linear variety of RMS and main Zernike polynomial coefficients of surface shapewhich is independent at the initial working condition of optical element. Finally, sensitivity matrix of Zernike polynomial coefficients changed by every supporting force is established, then estimation of surface shape due to support force adjustment in a new random working condition is made, the estimated result is similar to the finite element analysis surface shape, and the error of RMS is less than 0.3%, so the estimating method is able to guide the design and assembly of adjustable optic mount.