Abstract: The structure characteristic of the double damping system in a hydraulic rock drill was analyzed. The static balance position of the damping piston was determined by using the orifice flow law. The rebound velocity of the damping piston was derived from the transmitting principle of stress waves. The pressure equation of the 1st order damping chamber was deduced by considering the compressibility of hydraulic oil. The pressure of the 2nd order damping chamber was gained according to the equation of clearance flow rate. The motion differential equation of the damping piston was established based on these above foundations. The dynamic characteristics of the double damping system were simulated in Matlab. The movement laws of the damping piston were analyzed in different annular clearances. The relationships between annular clearance and the pressure in the 1st order and 2nd order damping chambers were discussed. The clearance-stroke coefficient was proposed,which was related to the ratio of annular clearance to maximum damping piston stroke. Simulation results show that the clearance-stroke coefficient is closely related to the damping system characteristics and has an optimal range. The mathematical model of the damping system was verified on the test platform.