By the kinematic and dynamic characteristics analysis of hydraulic excavator’s working mechanism system, the system was divided into three linear varying cross-section beam elements by using Finite-Element Method, then the dynamic differential equation of hydraulic excavator’s working mechanism system was established. The built equation better reflected the intrinsic relationships among the mechanism parameters, load parameters and its nonlinear dynamic properties, and it reflected the dynamic performances which the previous equations didn’t, so it laid the foundation of the further studies of the intrinsic relationships among the dynamics characteristics and mechanism parameters, load parameters of hydraulic excavator’s working mechanism system.