Abstract: The formation and creeping of the density current in the Xiaolangdi Reservoir are important indices of the water scheme and sediment regulation in the Yellow River. Previous studies have focused on the theory of density current in the Xiaolangdi Reservoir, real-time tracking on the operation process, and the analysis of surveyed data; however, they did not include related research on real-time prediction of the Xiaolangdi Reservoir and the tracking analysis of latest theoretical results. In this study, a field investigation, theoretical analysis, and prediction analysis were conducted to explain the process of water and sediment inflow from the Xiaolangdi Reservoir, during the period of discharging before the flood in 2018. The formation and creeping of the density flow were observed according to boundary conditions such as reservoir operation and reservoir siltation morphology. The plunging point formula of the density flow in the reservoir was predetermined, based on the velocity distribution. The results show that, in July 2018, intense scouring in the reservoir caused a significant amount of siltation to fall at the plunging area, thereby considerable decreasing the depth of clear water. This caused the plunging point to move downstream. The position of the plunging point moved ahead of the dam with the increase in water and sediment inputs, decrease of water level before the dam, and advance of the siltation in the reservoir. The calculated and measured results reflect on the changing process. These results can be used as a reference for the preparation and post-evaluation of reservoir water and a regulation plan for sediments. The results can also provide technical support for the operation as well as planning and design of reservoirs in sediment-laden rivers.