Aiming at the problem of the low efficiency of hard clay excavation，it is necessary to study the cutting mechanism of the clay. By using the method of numerical simulation，we establish the 3D model of cutting hard clay by cutter tooth，build the soil constitutive model by the nonlinear elastoplastic model，realize the numerical simulation of hard clay physical process based on the explicit integral algorithm and chip simulation unit damage failure criterion of hard clay，analyze the status of soil deformation in different times，and obtain the cutting force of the tooth. The results show that as the selected hard clay parameters，when the selected cutting thicknesses are 50mm，100mm and 150mm，the errors of numerical simulation and cutting experiment are 27%，9% and 5% respectively. Considering the inhomogeneity of soil，it is suggested to express the cutting load with the peak value of cutting force obtained from numerical simulation.