The dynamics model of the working device of hydraulic excavator is established on the basis of Equivalent Finite Element Method. The solution of the motive differential equation of the system is given through an example. The process of establishing the dynamic model is simple and with of high efficiency than the traditional methods. ADAMS is used to do the dynamic simulation of the mechanism and the simulation curves is given. It is proved that this method of establishing dynamics model is accurate by comparing the solution of the motive differential equation with the simulation curve of YW16 hydraulic excavator.