Based on the prototype data, the article theoretically analyzes and concludes that the water surface longitudinal slope of the stable braided river is greater than that of its single river . It analyzes characteristics of water flow and sediment motion and sediment transportation in the inlet reach of the braided river and their effects on fluvial evolution. According to the principle of sediment transportation equilibrium, a formula that describes the influences of the change of flow division and sediment division on its evolution is derived. That is to say, the evolution of the braided river is relevant to the upstream factors including sediment inflow, flow division and sediment division ratios of the braided river and their changes etc..