Abstract: A framework of damage mechanics in a related literature was introduced to geometric nonlinear analysis for the mechanical performance of beam considering material degradation.The relationships of elastic modulus and Poisson ratio before and after damage were derived, and the damage evolution equations based on the displacement function were proposed. Computer programs were developed to analyze geometrically nonlinear damage behavior of beam with update Lagrange method considering shear deformation effect. Numerical example demonstrated the damage effect on load-displacement curve and the effect of the damage threshold on the displacement of beam end.