In order to study the upstream distance of water backflow into the river channel, clarify the main control mechanism and dynamic laws of the tidal current limit, taking the Shiqi River, a typical tributary of the Nanjing section of the lower Yangtze River, as an example, 302 sets of runoff and tidal range conditions were simulated, and the variation range of tidal current limit in relatively dry years was quantified. The influence of runoff-tidal coupling on the tidal current limit was explored. The results show that the variation range of tidal current limit in the study area is from 0 to 7.64 km, and runoff is the dominant factor controlling the displacement of the tidal current limit. For every 1 m ³/s increase in runoff, the tidal current limit can be moved down by nearly 3.9 km, while the tidal range needs to be increased by more than 7 times to generate corresponding displacement in the tidal current limit. Affected by the downstream straight river regime, the confluence area of main and tributary rivers is the core stable zone of the tidal current limit. This terrain can effectively reduce the resistance caused by shallow shoals and bifurcations, allowing the tidal current limit to maintain a stable distribution with a high proportion(66%~75%) at both high and low top water levels. The distance from the tidal current limit to the estuary shows a stepped and non linear upward migration with the increase of tidal runoff ratio, which is different from the logarithmic change of the main stream. The tidal current limit in main stream of the Yangtze River is less sensitive to runoff due to its larger runoff and stronger tidal dynamics, with a variation range of 0.86 times that of tributaries.