Abstract: Vessels which navigate on the main flow channel can save energy.In order to more accurately divide the main and slow channels of the Yangtze River,this paper proposed methods of k-nearest neighbor algorithm and P-tile method.The improved P-tile method can avoid the main stream being too small by dynamic selection of P values.The k-nearest neighbor algorithm partitions the main and slow streams with channel hydrology information,such as depths and flow velocities.With the 1 1 sections'data of Yueyang and Jinkou measuring stations in Yangtze River,this study com-pared the partition results of these two algorithms.The results showed that areas of main and slow channels at the 10 out of 1 1 sections were close.A further comparison with the waterways at Longkou and Yanglinyan of the Yangtze River indi-cated the difference of the mainstream zone width estimated from the proposed algorithms was within 12% comparing to the actual,which can meet the requirements of the ship navigation.