The variation of the capacity of the energy accumulator is closely related to the operating process of the pipe rammer. Since the amount of oil required by the piston for forward and backward motions are different in the course of motion, the ideal status must be adjusted by change of the capacity of the accumulator, namely, change of the capacity of the accumulator must be coupled with the impact vibration of the rammer. The structural principle of the hydraulic pipe rammer is briefly described with the explanation of coupling between the accumulator and the capacity pressure of the rammer.