Cutter spacing is one of the important mechanical parameters that influence the cutting efficiency of tunnel boring machines (TBMs). A methodology was developed to optimize the cutter-spacing for efficient mechanical breaking, which improves the cutting efficiency of the jointed rock mass. In the beginning, 18 indentation tests were carried out on jointed granite specimens by varying cutter spacing, joint spacing, and joint set orientation, respectively. The cracking process of the jointed rock mass was captured by using a digital imaging correlation system clubbed with high-speed photography in real-time. Experimental results indicate that the joint plane facilitates horizontal displacement of the jointed rock mass, which causes more shear cracks develop across the joints. As a result, the efficiency of mechanical rock breaking of jointed rock mass becomes increases. Subsequently, a new term called crack propagation specific energy was proposed to determine the optimal cutter spacing, referring to the consumed energy per unit crack length. The optimal cutter spacing can be obtained from the limited experimental data based on crack propagation specific energy. It was found that the optimal ratio of cutter spacing to penetration depth s/p is 10. In addition, the horizontal displacement of jointed rock mass under the disc cutter favors the development of shear cracks, causing the increase of the optimal cutting spacing. The present investigation provides insights into the cutter design in jointed rock mass conditions, thereby making it possible for designers to adjust their design to enhance the TBM cutting efficiency.