Abstract: This paper, based on fields data, deals with the formation and evolution of the bar on Yellow River mouth. The following four substantial aspects are discussed: 1. The characteristic of the bar shape at Yellow River mouth, the distribution and change of the deposition of the bar, as well as the formative conditions of the bar are analyzed. 2. The huge sediment deposition at the place near the dynamic balance position between river and sea (that is the point where the the flow velocity tends to zero) is proved. However, due to the large differce between the discharges during the floods and dry during season in the Yellow River, the position of the dynamic balance point moves up and downward, the reach of the sedimentation increases, thus bar development continues. 3. The important characteristic of bar evolution is that it extends in flood season and eroded in dry season, and the change of the bar occurs in whole tidal flood and ebb processes. 4. Action of wind wave and tidal current on coast is that erosion occurs at upper part of the coast and deposition occurs at the lower part. The water depth of distinction line for erosion and deposition is about 7 5m.

Key words: runoff and sediment, stagnation point, sand bar, Yellow River mouth