Abstract: A finite-element model is established for the mold filling of co-injection, and a software package of numerical simulation was developed by means of Matlab and Visual C⁺⁺6.0. The numerical simulation of the relation was implemented between the core-skin-viscosity ratio and the formations of layer interface and moving melt fronts, and then a set of principles to guide the selection of the key molding parameters was put forward. The result of case study of numerical simulation is in good agreement with the experimental result given in some literature.