Abstract: This paper analyzes the mechanism of TBM rock breaking and provides the basic principle of disc cutter layout. The normal force, circumferential force, side force and inertia force probed are discussed and their corresponding mechanics formulas are presented. A balancing theory of forces acting on cutter head of TBM during rock breaking is put forward and a mathematical mechanical model of disc cutter layout is set up. A typical case with some imported TBM justifies the correctness of the mathematical mechanical model and its solution. Therefore an applicable approach to the layout of TBM disc cutter is developed.