The lower reaches of the Yangtze River is affected by runoff and tidal current, the hydrodynamic factors are complex. Based on the numerical model, the main characteristics of the tidal current in the Yangtze River are analyzed based in view of concept of the combination of the runoff-tide dynamic ratio. The characteristics of the tidal current limit under different runoff and tidal current conditions are studied. Based on the results of the model calculation of the Jiangyin reach on influence of terrain on flow distribution. The results show that the influence of runoff on the tidal range of the Yangtze River is greater than that of the tide. Due to the difference of the terrain, the runoff-tide dynamic ratio is smaller than that of the deep trough in the same section, that is, the tidal power is relatively stronger in shoal .