This research apply Preissmann's scheme and Double-Sweep Method to establish a model to solve Continuity Equation and Momentum Equation according to Saint Venant Hypotheses. The upstream boundary condition assumed to be designed discharge hydrograph, and the downstream boundary condition assumed to be water stage hydrograph of tidal. The base discharge at each channel is assumed with a small discharge. Assumed the flow is steady ,the initial depth of this model involves downstream boundary condition effect and is derived from downstream end using Standard-Step Method. This model mainly to simulate the horseshoe -shaped tube of open channel flow and base on the experiment result to prove the validity and accuracy of each parameter can be tested.