Cutting mechanism model is the foundation of analysis on rock drilling cutting force and cutting heat. The article reveals a new mechanism in medium hardness rock cutting model which based on a molar theory and the theory of crack propagation. The brittleness and ductility cutting exist in medium hardness rock cutting process. Take sandstone, marble and granite for drilling targets, micro-drilling experiments study is carried out. The results show that the rock cutting process means the rock damage taking place under the cutter torque and pushing force and resulting in small debris and large cuttings generated during continuous drilling cycle. Cuttings formation is mainly due to the extrusion deformation and crack generation expansion of rock. The experimental results and the rock cutting mechanism model display good consistency.