By using Lagrangian methods, the rigid body dynamics model was established for a planar 3 DOF controllable excavating mechanism. Based on the system generalized forces of the three driven bars, the control strategy of the semi closed loop control system was studied. A Fuzzy PID dual mode controller was introduced and analyzed by using fuzzy algorithm based on the mathematical model of mechanism driven components AC servo motors. The simulation results indicate that the proposed controller has better performance in overshoot, adjusting time, rise time and anti interference ability, which can satisfy the control requirements of the system.