Based on tensile and shear failure mechanisms of rock breakage, a synthetic failure hypothesis has been proposed. With the establishment of the non-collaborative and collaborative cutting volume model, a theoretical model is deduced for the specific energy of tunnel boring machine (TBM) cutters working in the collaborative cutting mode. The relationship between the crack length and penetration is established by a discrete element model (DEM). A theoretical formula of optimal cutter spacing is then derived by the fitting method. Additionally, indentation experiments are conducted the cement simulation material as cutting objects using the TBM rotary cutting test platform. The relationship between the hob penetration and the crack length of the rock is obtained, and then the simulation results are verified by experimental results. The broken-rock volume is counted through 12 cutting experiments with different penetrations and spacings between cutters. Then the specific energy consumption fitting curve is obtained, which verifies the conclusion of optimal cutting spacing equation. The research results show that the characteristics of rock properties and the structure of the hob are comprehensively considered in the optimal cutter-spacing calculation formula of the full-face rock TBM, which potentially has wide applications. When the cutting spacing is greater than the collaborative spacing, the shear failure mechanism plays a dominant role in rock-breaking process. However, when the cutting spacing is less than the twice lateral crack length, the tensile failure mechanism has more significant effects on rock breakage. As the penetration increases, the optimal spacing increases while the optimal S/P decreases.