The sedimentation impacts of incoming water and sediment from the six observation stations in the middle reaches of the Yellow River on Sanmenxia reservoir and its downstream area are studied by using the sediment discharge balance method. The downstream sedimentation index induced by incoming sediment from the middle reaches of the Yellow River, the area rich in sediment and coarse sand, and the source area dominated by coarse sand are analyzed by using profile method. Results show that incoming sediment from Longmen station has the greatest contribution to sedimentation in the lower reaches; incoming sediment from Huaxian County, Hejin, and Zhuangtou stations has smaller contribution; and incoming water and sediment from H eishiguan and Wuzhi stations are favor of downstream scour. Analysis by profile method indicates that the amounts of sedimentation in Sanmenxia reservoir and its downstream area are 0. 25, 0. 257, and 0. 329 hundred million tons for one hundred million tons of sediment from the middle reaches of the Yellow River, the area rich in sediment and coarse sand, and the source area dominated by coarse sand, respectively. The coarser sediment, the greater contribution to downstream sedimentation is.