Abstract: Using large reservoirs on the mainstream of the Yellow River to adjust water and sediment, or so-called water and sediment regulation, will transport a huge amount of sediment into the sea in a very short time. It is an important window period to trigger the hyperpycnal flow in the estuary. This paper is devoted to the forming process of the estuarine hyperpycnal flow with daily water and sediment data from the Lijin station and the tide and wind field data from the sea in addition to laboratory simulations. The research has reached following conclusions. The hyperpycnal flow will be formed when the suspended sediment concentration of the runoff input exceeds 29.0 kg/m³. Under the influence of the tidal current, the moving direction of the diluted water tongue shifts from northwest to southeast over time, and ends on the west side of Laizhou Bay. The surface plume has a wider influence range, and the transportation path goes from the northwest of the estuary-remote bank (northeast area of the estuary) up to the west side of Laizhou Bay with less suspended matter carried and the suspended matter can hardly be transported to Laizhou Bay. The bottom hyperpycnal flow moves from the northwest of the estuary to the west along the estuary (east) with a small amount of suspended solids reached at the Laizhou Bay. Factors influencing the water structure mainly include runoff injection, tidal strain and stirring, among which the tidal strain and stirring is 2～3 order in magnitude (10²～10³) higher than the runoff injection.