At present, water-assisted injection molding is suitable for producing hollow or part of hollow plastic products. The research of viscoelastic properties can provide an improved understanding about the influence of related process parameters on the filling process of water-assisted injection molded parts. In this paper, based on the open source code of OpenFOAM, an analysis module was constructed for the three dimensional isothermal flow of multiphase fluid which can be subscribed as Giesekus viscoelastic model, by the object oriented program and the volume of fluid method(VOF). The filling process of the water-assisted injection molding was simulated by the solver called viscoelastic InterFoam. The result agrees with the experiment of Polynkin, which shows that with increase of the injection rate, the residual wall thickness of melt and penetration distance of water will be increased. That can be analyzed according to viscoelastic properties of melt.