Abstract: The regulation mechanism of Longyangxia and Liujiaxia Reservoir on runoff and sediment process is one of the key scientific problems to recognize flow and sediment variation in the Yellow River and optimize the reservoir operation modes. According to the located-observation data of water and sediment in Longyangxia and Liujiaxia Reservoirs, this paper analyzes the regulation and effect of reservoir to runoff and sediment process and reveals the controlling mechanism for the relationship between water and sediment. The main understandings achieved are:① The regulation of large reservoir operation on runoff sediment is related to the operation mode of the reservoir. The pluriennial regulation of Longyangxia Reservoir results in the linear regulation of runoff and nonlinear regulation of sediment transport. The incomplete annual regulation of Liujiaxia Reservoir results in both linear regulation of runoff and sediment; ② Reservoir operation on the Regulation of runoff and sediment has a strong disturbance to the downstream water and sediment relation. It broke the sediment law under natural conditions of the river formed by long-term automatically adjustment and changed the constitutive relation of runoff sediment; ③ The effects of large reservoir operation on the dynamic mechanism for the relationship of water and sediment regulation mainly reflect in adjusting the dynamic equilibrium conditions of sediment-laden flow in which the shear stress decreased in the process of flood propagation. It makes the critical Froude number increase, resulting in the sediment-laden flow by siltation to increase gradient ratio to meet the critical water shear stress requirements of the Froude number state and reach the steady state of sediment-laden flow to stress the relationship between water and sediment change accordingly.