The study of rock fragmentation mechanism by TBM cutters is the key to guide the development of TBM cutterhead and the selection of cutterhead in different geological conditions. The formation, propagation and coalescence of cracks in rock masse is the precondition of understanding the rock fragmentation mechanism by TBM cutters. Therefore, the study of rock fragmentation mechanism by TBM cutters is significant for engineering application. At present, the study of rock fragmentation mechanism mainly concentrates on the single rock. However, the complicated geological conditions are often encountered during the excavation of tunnels by TBM. In this paper, a discrete element method is employed to study the fragmentation mechanism of the composite rock mass. It could be obtained from the numerical results that the propagation of microcracks initiated from the interface leads to the coalescence of Hertzian cracks between two cutters, and the formation of chipping. Therefore, under certain conditions, the formation of chipping in composite rock mass by TBM becomes easier than that in the single rock.