Abstract: In order to research the rock breaking mechanism of TBM cutter in tunneling construction under different confining pressures, based on the 2-D discrete element method, a simulation model for breaking rock by two TBM cutters is established by using UDEC. On such a basis, a series of numerical tests are designed. The whole process of crack production and propagation is successfully simulated under different confining pressures as well as different cutter spacings. The simulated results show that there are four kinds of breaking patterns of rock which are related to the confining pressures and cutter spacings under the action of TBM cutters. Breaking patterns both affect the breaking efficiency and the rock fragment size. Along with the increasing confining pressure, the optimum cutter spacing increases, but when the confining pressure exceeds a certain value, it decreases inversely.