Abstract: In order to explore variation law of hydrodynamics in tidal branching channels, this paper investigates the changing features and mechanisms of ebb-tide and flood-tide partition ratios within two branches of waterways of the Chengtong reach of the Yangtze River under varying runoffs and tide forces by using measured hydrologic and topographic data during the wet season in 2004 and the dry season in 2005.Results show that variations of ebb-tide and flood-tide partition ratios in the two branches of the waterways under varying relative tidal level and a certain runoff discharge are determined by the decline-growth rates of tidal current velocities in the two branches, while the variations of the ebb-tide and flood-tide partition ratios in two branches of the waterways under varying runoff discharge and a certain relative tidal level depend on paths of ebb-flow axes, deflecting effects on the flow of alongshore nodes, flood tidal levels and flood tidal velocities under different confrontation strengths between runoffs and tides.