Abstract: Based on the fact that the information propagation distance in one time step can not be greater than the length of N (Courant number) grids, a new approach, namely overlapping projecting method, is proposed using the time delayed boundary condition. According to the method, a coupling model is developed to join the calculation domains of 1 D and 2 D numerical models. Comparison of the calculation results of the coupling model with those of 1 D model shows that the coupling model can simulate unsteady flow with tributaries and is stable, reliable and practical.