In rock drilling process with percussion-rotary drill rotating drilling tool bears two toques: dynamic torque which drives drilling tool into rock mass, and resistance torque of rock mass. The drilling torque must be greater than resistance torque moment so as to operate the drill stably and normally, on the basis of analysis on resistance torque constitution and its influence factors, calculation formulas for related parameters and resistsnce torque were established. For this purpose according to bit structure parameters, working state and rock condition predetermination of driving torque necessary for percussion-rotary drilling provides reliable theoretical basis for the design of auxiliary screw rotation mechanisms and in-the-hole drilling tools as well as choice of rotary motor and working state parameters, etc.