Abstract: River channels are often connected each other to form a reticular river channel or divided into two or more branches in plain areas.The numerical methods based on the Preissmann implicit scheme for flow simulation in these channels are discussed in this paper.After the derivation of the Saint-Venant Equations,water level and discharge of each cross-section are derived by the recursion method and the principle of water balance.The method is proved to be satisfactory when applied to the flow calculation in reach from Jiangyin to qnglonggang and Yanglin in downstream of Yangtze River.