<p>This paper develops a rock drillability index to determine rock strength by interpreting percussive pressure, penetration rate and rotary speed etc. drilling performance parameters in percussive-rotary drilling. Orthogonal experiments were initially conducted by a drilling processes monitoring setup to establish a database of percussive-rotary drilling performance. This database involved drilling performance parameters for 13 types of rock under 7 levels of percussive force, 7 levels of thrust and 5 levels of rotational speeds for examining the relationship between drilling performance parameters and penetration rate. A new rock drillability index was subsequentially proposed based on the nondimensionalize processing to eliminate the influence of drilling parameters on penetration rate. Experimental results demonstrate a negative linear correlation between penetration rate and feed pressure, but independent of the rotation speed. Moreover, the relationship between percussive pressure and penetration rate characterized by an upward-opening parabolic curve. Additionally, each rock type demonstrates a unique radius of curvature, but generally similar axis of symmetry. The proposed rock drillability index shows a great potential in hard rock strength determination via percussive-rotary drilling, and its accuracy, proactive assessment, and real-time capabilities have been validated in practical tunnel engineering.</p>