Inflow/outflow is one of the main styles of water currents in lakes, which affects pollutant diffusion and transportation?sand scouring and sediment, and lake density field variation through water exchanging. A numerical model of two-dimension and open-boundary is proposed in accordance with the features of Hongze Lake in this paper. The model is used to study the course of in-flow/outflow, which is engaged in numerical experiment for the characteristics of Huaihe flow through the lake.