Torsional impact drilling is a new technology which has the advantages of high rock-breaking efficiency and a high rate of penetration (ROP). So far, there is no in-depth understanding of the rock-breaking mechanism for the ROP increase from torsional impact tools. Therefore, it has practical engineering significance to study the rock-breaking mechanism of torsional impact. In this paper, discrete element method (DEM) software (PFC2D) is used to compare granite breaking under the steady and torsional impacting conditions. Meanwhile, the energy consumption to break rock, microscopic crushing process and chip characteristics as well as the relationship among these three factors for granite under different impacting frequencies and amplitudes are discussed. It is found that the average cutting force is smaller in the case of torsional impact cutting (TIC) than that in the case of steady loading. The mechanical specific energy (MSE) and the ratio of brittle energy consumption to total energy are negatively correlated; rock-breaking efficiency is related to the mode of action between the cutting tooth and rock. Furthermore, the ROP increase mechanism of torsional impact drilling technology is that the ratio of brittle energy consumption under the TIC condition is larger than that under a steady load; the degree of repeated fragmentation of rock chips under the TIC condition is lower than that under the steady load, and the TIC load promotes the formation of a transverse cracking network near the free surface and inhibits the formation of a deep longitudinal cracking network.