Efficiency and stability are the key factors affecting the simulation results of three dimensional （3D）plastic injection molding with the finite element method (FEM).In this paper tetrahedral element based on the Galerkin method was used for the 3D flow simulation of viscous incompressible plastic melt.As the scheme of P1/P0 tetrahedral element (linear velocity and constant pressure）produced unstable results

a new P1/P0 tetrahedral macro-element was used for the discretization of the spatial domain

which was stable and could reduce the CPU time by decreasing the degrees of freedom of the equation.As a numerical test

developed flow with constant viscosity in the cylindrical geometry was used to compare the accuracy of the results given by P1/P0 tetrahedral macro-element and P2/P1 tetrahedral element (quadratic velocity and linear pressure).Besides

the higher efficiency of the P1/P0 tetrahedral macro-element was obviously witnessed by another test of simulating the filling process of the fluid with constant viscosity in cylindrical geometry.In the last part of this paper the scheme of P1/P0 tetrahedral macro-element was used for the simulation of plastic injection molding in filling stage.