Abstract: This study focuses on the braided reach of the Lower Yellow River (LYR), which features abrupt and random river regime evolution. Studying such river regime stability variations and dominant controlling factors is crucial for addressing challenges such as channel braiding and flood control. Using the cusp catastrophe theory, this study takes thalweg migration intensity and flow scouring intensity as control variables to quantitatively analyze river regime stability in a typical braided reach before and after the operation of the Xiaolangdi Reservoir. The results show that river regime stability exhibited an overall increasing trend from 1992 to 2020. Before 2006, the river regime had been in a persistently unstable state, whereas after 2006, it shifted markedly to a stable state under water-sediment regulation, demonstrating the remarkable effect of altered water-sediment regimes on enhancing channel stability following the operation of the Xiaolangdi Reservoir. It was also found that the stability coefficient was significantly positively correlated with average scouring intensity and negatively correlated with thalweg migration intensity, where the discharge and sediment conditions constituted the dominant controlling factors. The results also indicate that after reservoir operation, average scouring intensity continuously increased, while thalweg migration width and intensity decreased significantly, leading to a substantial reduction in channel braiding. In addition, sediment concentration in flood seasons was the key factor affecting river regime stability as higher sediment concentration led to lower scouring intensity, greater thalweg migration intensity, and thus decreased river regime stability. Based on these findings, this study reveals the mechanism of stability variations in braided channel evolution driven by water-sediment regime changes, and provides theoretical support for water-sediment regulation and systematic governance of the LYR.