The dynamic performance of hydraulic beam pumping units was analyzed in this paper by using the theory of mechanical vibrations. The house-head movement of the pumping unit is mainly uniform, except the alternation period of upper- and down-strokes. Under the action of the house-head movement, the vibration of the system, the sucker-rod and, furthermore, the dynamic stress will be induced. The results indicate that the movement of the downhole pump includes two parts. One is the movement following the horse-head. The other is the dynamic response excited by the support movement. When the parameters of the system are selected reasonably, over-stroke of the pump will appear. This is because the movement of the hydraulic piston obeys a particular law. The maximum displacement increases, and the maximum dynamic stress decreases with depth. The changing of maximum dynamic stress with depth obeys quadratic law.