Abstract: The important parameter of water sediment regulation is the threshold of grouping sediment concentration about floods scouring and silt ing relative balance in hyperconcentration river.The study on the threshold of flood grouping sediment content and incoming sediment coefficient is based on the more 200 flood data measured in the lower Yellow River The data shows that the riverbed scouring and silting can obtain relative balance when the floods have determinate grouping sediment content and incoming sediment coefficient.For the floods with the suspended sediment consisting of thin silt,the crit ical sediment content of watercourse scouring and silting are influenced by unit discharge of main riverbed,the average maximal daily discharge,the least bankful discharge in the lower Yellow River beginning from Huayuankou and the weight percentage of riverbed sediment whose median diameter d₅₀ is bigger than 0.05mm; For those with the suspended sediment consisting of the middle coarse silt,the critical sediment content of watercourse scouring and silting is irrelative to the weight percentage of riverbed sediment whose median diameter d₅₀ is bigger than 0.05 mm; For those with the suspended sediment formed by the coarse silt,the critical sediment content of watercourse scouring and silting are influenced by the unit discharge,the average maximal daily discharge,the least flat beach discharge in the lower Yellow River beginning from Huayuankou and the weight percentage of riverbed sediment whose median diameter d₅₀ is less than 0.025mm and bigger than 0.05 mm The similar sediment content floods have the critical grouping incoming sediment coefficient,which means that different discharge can bring forth different scouring and silting outcome.The result can provide evidence for water-sediment regulation of Xiaolangdi reservoir on the Yellow River,and enrich the river dynamics research.