Based on the model experiment and theoretical analysis， we discuss the mechanism of Karman vortex produced from the arc spur dike and the reason of scouring behind the spur dike, and discuss the relationship between the Karman vortex and the scouring behind the spur dike. The distribution of the scouring area and the deposition area behind the dam depends on the relationship between the velocity which is synthesized by the linear velocity on the edge of the Karman vortex and the approaching velocity of the flow and the starting velocity of the sediment behind the dam.