Abstract: Fine grains in loose soils can be erosion and re-deposition by seepage which can change the grain series and micro-structure of the soils. Accordingly the macro properties such as strength and modulus can be changes. In this paper test based on nuclear magnetic resonance instrument observation and simulation based on pore network model were carried out to analyze the effects of the migration of fine grains on the soils macro-properties. It is shown that fine grains are eroded and migrated along depth with pore water flow and most of them are assembled at the middle of the soil layer, which causes the porosity at the diddle layer is obvious smaller than that of the upper and lower layers. During seepage, pore water cannot flows through the whole area. In fact, at the upper, lower and middle layers, there all exist small or large areas which are not filled by water. Simulation shows that the area of the middle layer spread by water and the flow channels are the least, therefore under the same flow rate the pore water pressure is higher, so the changes of the strength and stiffness in the middle layer are the largest.