Abstract: In order to enhance the performance of undercutting cutter in breaking hard rock and reduce rock breaking load, the numerical model for hard rock breaking by undercutting cutter at precut slot has been created to investigate how various combinations of cutting thickness values and slotting parameters (slot spacing, slot depth) will affect the rock breaking performance of undercutting cutter, and the mechanism by which precut slot affects the rock breaking process of undercutting cutter has been revealed. As the study results indicate, when the slot spacing is equal to the cutter diameter, the rock with precut slot can be equivalent to a rock with unconfined compression, and the cutting force is reduced by over 30% as compared with cutting an unslotted rock. Besides, the matching relation between cutting thickness of undercutting cutter and slot spacing will significantly affect the specific energy of rock breaking. To achieve the optimum specific energy of rock breaking, the cutting depth and slot spacing are 20 mm and 100 mm respectively. In addition, the relation between fractal dimension of rock debris and specific energy of rock breaking has been established, i.e. the specific energy consumption increases linearly as the fractal dimension increases