Based on the numerical manifold method (NMM), the numerical simulation of metal plastic forming process was studied. The basic theory of NMM was introduced, which divided the solution domain by mathematical grid and physical grid, and classified and processed the boundary manifold elements in the deformation process. By tracking the intersection of motion interface and mathematical grid, thirty-two kinds of boundary manifold element types were constructed. Based on the characteristics of NMM approximation function construction and the standard rectangular mathematical covering grid, Gauss integral method was proposed for numerical integration of manifold elements. Combined with the Gauss integral point and their weight coefficient, the integral points and the irregular integral area were mapped and the integral calculation was completed. Different from the finite element method, the partial derivative mapping and Jacobian matrix calculation were not required for NMM, only the coordinate transformation and area mapping were needed to deal with the numerical integration of irregular shape elements. The research results provide a theoretical basis for the application of NMM in the numerical simulation of metal plastic forming.