In order to accurately describe the dynamic model of hydraulic excavator’s mechanical arm , the modal functions are adopted to describe elastic deformation of the mechanical arm ,Lagrange theorem and the principle of virtual work is used to establish rigid-flexible coupling nonlinear dynamic equations of arm frame system. The dynamic equations are numerically solved by MATLAB, the rigid-flexible coupling model of hydraulic excavator’s mechanical arm is established and simulated by the simulation software ADAMS and NASTRAN , it is showed that the modeling method of dynamic equations adopted in this paper is correct by contrasting the both results. The modal and dynamic response is calculated by applying the numerical solution method ,the sensitivity of the first natural frequency for the Geometric parameters related is solved, the main modal parameters are analyzed which influence dynamic characteristics of mechanical arm, the basis is provided for the further optimization of excavator structure and motion error control.