Abstract: In order to investigate the effect of rock breaking with a PDC bit under axial percussion and torsional percussion,and the influence with different drilling parameters and percussive parameters,a composite (torsional and axial) percussion experimental device was designed.Then composite percussion experiment and a conventional rotation experiment were carried out with sandstone,limestone and granite samples to analyze the percussion acceleration effect of a PDC bit with drilling parameters such as WOB,RPM,percussive force and percussive torque under composite percussion.Experimental results showed that WOB and RPM are the two main controlling factors that would influence the efficiency of rock breaking..They also demonstrated that drill cutting size under composite percussion is larger compared with conventional rotation,so the efficiency of rock breaking is higher.As percussion force and percussion torque increase,the rock breaking efficiency increases,with the ROP could be increased by up to 50%.Studying the influence law of composite percussion drilling parameters on rock fragmentary efficiency helps design and optimize the performance parameters of composite percussion tools.