Abstract: In recent years, the Sixianjiao channel at the apex of the Pearl River Delta has undergone substantial changes in diverting the flow between the Xijiang River and the Beijiang River, which in turn has dramatically affected the flood prevention and control strategies and the water supply configurations in the delta.Therefore, research into the flow diversion process of the Sixianjiao channel is essential for the effective governance and control of flow diversion throughout the delta.The aim of this study was to elucidate the pattern and mechanism of the adaptive regulation of the flow diversion of the Sixianjiao channel using physical and mathematical models and theoretical analysis of a combination of hydrological and topographical monitoring data for the Sixianjiao region since the 1960s.The results show that the Sixianjiao channel acts as a natural regulator during floods, maintaining a steady distribution ratio of floodwater between the Xijiang and Beijiang rivers.The flow ratio between Makou in the Xijiang River and Sanshui in the Beijiang River becomes increasingly stable as the flood intensity increases, typically with a ratio of approximately 3∶1.The topography of the approximately 7 km long river segment downstream of the Sixianjiao region is the primary factor controlling the adaptive regulation of the water diversion during flood periods.As the upstream flow increases, the ratio of the flow cross-sectional areas of the Xijiang and Beijiang rivers tends to approach a constant value due to the constraint by the embankments on both sides of the two rivers, thereby leading to stabilization of the water diversion between the two rivers.During the low-flow period, the tidal backwater in the downstream region or the operation of sluice gates leads to changes in the hydraulic gradient, which in turn triggers changes in the water diversion in the Sixianjiao channel.The diversion ratio is more responsive to changes in the water body downstream of the Beijiang River compared with the Xijiang River.