After the operation of the Xinglong Water Control Project at Hanjiang River, the low water level near the downstream of the dam has significantly decreased compared to that before the construction of the dam. Under a flow rate of 500-800 m³/s, the cumulative water level under the dam has decreased by 2.47-2.55 m in 2021 compared to that before the operation. Such water level decrease had adverse effects on the safety and efficiency of the Hydro-junction. Based on the observation data of hydrology, cross section and underwater topography, the causes of low water level decline were analyzed. The results showed a limited sediment retention of the Xinglong Water Control Project. Reduction of sediment flux throug Huangzhuang was an important factor enhancing riverbed erosion below Xinglong. The decrease in the probability of dry water reaching the beach during the year caused by high water level regulation, the channel regulation and protection engineering were critical factors of "beach sedimentation and channel erosion" in the process of strong erosion. The direct reasons for the decrease of dry water level in the Xinglong near dam section were the erosion of the low water channel downstream of the dam and the decrease in the lowest water level in Hankou. The random forest algorithm analysis showed that the most critical factor affecting the changes in water level under Xinglong Dam was the sharp decrease of sediment flux at Xinglong Station. In addition, the waterway regulation project and the boundary conditions of the riverbed also played a critical role in the decrease of low water level. The riverbed composition at the Xinglong Dam site and downstream reaches was relatively fine. Although the riverbed was severely eroded from 2012 to 2022, there was no coarsening of the sediment composition. Therefore, it was expected that the riverbed will continue to be eroded and cut down. The low water level has not yet reached a stable state.