The vibration dynamics of the lower tup slightly moved hydraulic hammer is researched for the first time by means of the method of mechanical dynamics combined with digital simulation. The blowing process and energy transformation of hydraulic hammer is analyzed. The mechanics model and the mathematical model of vibration of the hammers are established. As an example the digital simulation of vibration of 25KJ hydraulic hammer is performed. The variation curves of vibration of the lower tup and the base are obtained. It is shown from comparative research that the vibration of this new kind of hammer is much less than that of steam-driven anvil hammer. Some measures to eliminate vibration produced by the hammer furthermore are put forward.