In order to guide indoor model experiments or numerical simulation directly and engineering practice indirectly, the process of rock cutting was simulated by using the DEM software where the reasonable contact model for rock summarized from the micro bond contact experiment was implanted. The influence of cutter width and rock sample size on the rock fragmentation was investigated. The results show that cutter width is too small to keep the stability of the actual TBM construction. However, large cutter width in small sample size has a significant boundary effect which affects the test result. In the indoor model test, there exists a critical ratio of the cutter width and the rock sample width (relative ratio). If the relative ratio in the test is less than the critical ratio, the boundary effect can be reduced during the test.