Abstract: The study of maintaining the critical channel slope of the middle-water channel in the tail reach of the Yellow River Estuary can provide effective parameters for the estuarine channel regulation project, and is of great significance for maintaining the stability of the flow path of the Yellow River Estuary. This paper explores the critical channel slope of the middle water channel in the tail reach of the Yellow River Estuary. Firstly, on the basis of the research results on the principle of the minimum work of the river bed evolution, the mathematical expression of the minimum work average velocity of the reach is derived, and the functional relationship between the minimum work average velocity of the reach and the inflow and sediment conditions, the flat discharge of the tail reach and the channel slope in the quasi-equilibrium period is calculated and analyzed. Finally, the channel slope range which should be controlled to maintain the flat stream discharge in the middle water channel of 4 000 m3/s in the sink section of the Yellow River estuary is estimated, according to the established functional relationship.The results show that, the correlation coefficients between the minimum average velocity of the tail reach of the Yellow River Estuary and the average flood season discharge, sediment concentration, sediment transfer coefficient and the plain discharge in the same period are all more than 0.9. According to the calculation results, the critical channel slope range of the middle water channel in the tail reach should be between 0～0.087.