During the construction of the cutter suction dredger,the effect of the cutter load on the construction is extraordinarily significant.As the process of the cutter acting on the rock and soil is 3D,and the deficiency of traditional 2D cutting theory in the load analysis of the cutter,the three dimensional cutting theory applicable to the cutter is made.Based on the 2D cutting theory,the 3D mechanical model of rock and soil for the analysis of the cutter load is proposed,and the 3D coordinate matrix of all the teeth of the cutter is established.Using coordinate transformation method to judge the cutting forces,the expression of the cross load,ground load and thrust load of the cutter are given.The example shows that the proposed method can be used to calculate the crosswise load,the ground load and the thrust load of the cutter.As the design of the cutter and the safety construction of the cutter suction dredger,the theory will play a guiding role.