Based on the channel construction project of the north channel for the southwest water transport，we analyze measured data of many years on the water level of multiple existing reservoirs，and get to know that the comprehensive roughness will be reduced after completion of the water conservancy，and the ratio of the channel’s comprehensive roughness after completion of the water conservancy to the natural river is 0.862.Using the results to calculate the designed minimum navigable stage of Yangxi and Meilin reservoir，it shows that the calculation result accords well with the natural conditions.