A new model has been developed for studying the mechanism of infiltration destruction by contact scouring between the hardpan and grid. The permeability of coarseness gap that formed by different lithology is substituted by the permeability of slippery cranny, and the permeability of the stratum can be expressed by a combination of the permeability of gap and sand and gravel. The starting speed of the fine sand can be used as differentiation criterion. To calculate the fine sand moving by stabilization flow theorem in different time part, the fine sand near the orifice flows out of the stratum at the beginning. At first the permeability coefficient near the orifice becomes larger, and then it developes inside. It will develope rapidly in the direction perpendicular to tho bank, and even forms a concentrate leakage passage.