Altitude axis precision is important to the performance of a large aperture telescope. At practice, the tested result can not reach the system requirement, but the performance is well enough,so there must be error factors caused by centerpart deformation that can be eliminated in this system.To improve the accuracy of an altitude axis precision model,the centerpart, a main part effeting altitude axis precision is analyzed by the Finite Element Method(FEM), and then the curve of altitude axis max deformation versus pitch angle is obtained.After analyzing the influence of deformation on the axis precision,a modification method is proposed,which uses a compensation method to add the tested results to the calculated results. By this way, the PV error and the RMS error of axis shake are reduced from 2.42″ to 0.95″ and from 0.7″to 0.3″,respectively.The method can improve the accuracy of the system precision model ,and can provide theoratical basis for design and test of an even larger telescope.