Abstract: A branched river exists widely as an important river form. When the river channel is wide, the cross section of the river channel is likely to form a compound section. In the evolution of the whole river morphology with the central bar, the flow movement often plays an extremely important role in the diversion section of the head of central bar. The distributary action of central bar superimposed on the interaction of compound cross-section floodplains and main channel will jointly affect the distributary and sediment separation, and ultimately affect the evolution of the whole channel morphology. In order to explore the characteristics of the flow motion of the head of central bar on the evolution of the compound braided channel, an experiment of a straight compound braided channel eroded by clear water is designed in this paper. The test results show that,（1） the vertical velocity distribution basically meets the logarithmic distribution in the compound section in front of the central bar in the straight compound braided channel；（2） the flow velocity of the compound braided river is mainly affected by the braided morphology；（3） the shape of compound cross section is the main influencing factor of the flow velocity change in the main channel and floodland at the bifurcation；（4） in the realignment and protection of compound braided channel, we should pay more attention to the scouring at the head of the central bar and both sides and the siltation and scouring caused by the difference of flow velocity between the two branches；（5） When the discharge increases to a certain value, the section shape of compound braided channel can be simplified to a simple rectangular channel for research.