On the basis of the transient stress wave propagation theory of one-dimensional vibration elastomer,a model is constructed to calculate the coaxial longitudinal collision of double flexible rod during the impact of hydraulic hammer drill rod on the corresponding piston.Then,the displacement-time transient response impact function of each impact part is solved by means of the eigenvalue expansion method of transient wave function,and the stress wave propagation rules of both the drill rod and the piston as well as the dynamic characteristics of each crosssection of both the drill rod and the piston are analyzed.Furthermore,the transient dynamic responses of impact parts with different drill rod lengths and diameters as well as with different initial speeds of piston are discussed.The results indicate that ( 1) drill rod length slightly affect the impact force and a short drill rod may result in a fast rebound of the piston; ( 2) with the increase of drill rod diameter,the impact force amplitude increases and the crushing ability improves,which means a relatively low energy efficiency; and ( 3) with the increase of the initial piston speed,the impact force proportionally increases and the crashing ability of hydraulic hammer greatly improves.These results can provide a theoretical basis for an optimization of the structure and performance of hydraulic hammers as well as a reasonable adjustment of construction parameters.