At present the research on water damage mechanism of spur dike are more confined to dike head, but that a large number of water damage phenomenon happened in the dike surface and downstream slope is found through analysing the field investigation and experimental phenomenon. This article first simulates the boundary condition on water damage of dike surface and downstream slope, and then observe the process of washout, so that reveals its mechanism. When the dike body is riprapping, destructive flow condition of recession period and the next flood season, leading to the unstable of dike body and the large pressure difference between its front and back, will cause dike surface and downstream slope be destroyed. When the dike body is riprapping and dike furface is concrete hinge, stream of overflowing dam with an apparently downward acting force causes the serious erosion on the downstream side. With the development of the scour process, local large permeable rate basis area will be scoured much more seriously. When the dam surface is unable to bear the force down to the dam crest from the downward water flow, it will be collapsed.