Abstract: Based on seasonal bathymetry observations and high-resolution validated FVCOM model, the North and South Passages in the Yangtze River Estuary are studied as an integrated system to determine the dynamic mechanism controlling their flow diversion ratios in recent ten years. The variations of the sectional areas and averaged velocities are analyzed to reveal the interannual patterns of the diversion ratios between these two Passages. The results show the overall trends of the section area and velocity are both decreasing in the North Passage. However, there is an opposite pattern shown in the South Passage. Therefore, the flow diversion ratio decreases in the North Passage while it increases in the South Passage. Although the variations of section areas and flow strength jointly lead to the changes of diversion ratio, the quantitative linear correlation analysis indicates the change of sections areas has more dominant effect on diversion variation than the change of flow velocity.