Based on the view that the combination of cutting tools can produce pre-break zone in rock, this paper quantitatively studies the relationship between the depth of pre-break zone and the cut depth of cutting edge. The results show that the pre-break zone can contribute to reduce rock strength and rock fragmentation energy, which is also verified by the field test results. It gets the following two academic views: there are still pre-break zone, cutting and micro-cutting rock breaking modes when drilling Ⅶ~Ⅷ-class hard rock with the impregnated diamond bit; the view “the bigger of pre-break zone, the better of drilling effect” is not correct.