Present gas-assisted injection molding simulations are all based on either a midplane model or a 3D model, in which second modeling is unavoidable for a midplane model, and a 3D simulation needs a full-scale three-dimensional discretization of parts leading to unsustainable computing time and unstable numerical analysis. In this paper, surface model based modeling and numerical simulation of gas-assisted injection molding are proposed. By taking the influence of gas penetration on melt flow as boundary conditions of the melt-filling region, a hybrid control-volume finite element/finite-difference method (CV/FEM/FDM) similar to conventional injection molding simulation is employed. The gas penetration interface within the gas channel is solved by applying the matching asymptotic expansion method, which educes an analytical model of the gas penetration thickness ratio. A technology of generating gas-channel mesh semiautomatically is put forward, which combines selecting the path of gas channel manually and calculating the parameters of gas nodes automatically. The second modeling is thus avoided. The proposed model and simulation are verified by comparing with the experiment.