To investigate rock breaking mechanisms induced by cutters of tunnel boring machine (TBM) under a biaxial state, TBM indentation tests were conducted on granite specimens using a triaxial testing platform. Then, detailed morphological measurement was carried out to analyze the morphology and volume of the fractured surface using a surface profilometer. It is found that the increase of differential stress results in the increase of consumed energies, the growth of groove volumes and the efficiency of indentation when the Sig_1 (i.e., the smaller confining pressure) was fixed. The energy of indentations increases as the differential stress decreases, but the groove volumes are decreased, and thus the indentation efficiency is restrained by the decline of the differential stress when the Sig_2 (i.e., the bigger confining pressure) was fixed.