The rock broken mechanism of percussive-rotary drilling was discussed preliminarily by fracture mechanics method. There are a lot of joints and micro-fissures in rock matrix. Shock forces make fissures in rocks nearby bit fracture, break and form crater let, make micro-fissures in rocks a little far from bit expand, joint and even break, and make rocks farther from bit generate new micro-fissures. Rotation enlarges the crater lets formed by shock along circumferential direction, and changes the positions of diamond particles on bit.