The solidification model is vital to the simulation of casting process with mushy zoned involved. The author has built a multicomponent model for solidification which is derived from the principles of fluid dynamics. The model has been realized on the commercial CFD package, in which the solid and liquid of metal are united in one frame. In this model the flow behavior of solid is constrained by increasing the viscosity, the latent heat and the buoyancy are also considered, and the key point is the effect of solid fraction on the flow in the mushy zone. The details of flow can be captured even in the mushy zone by using this model.