Based on the theory of soil hydrological interface process, studies the transport of matter between soil and other environmental medium, especially the agricultural non-point sources pollution (ANPS) phenomenon caused by the transport of agricultural chemical in soil among different medium. Take the small typical watershed for example, with mathematical models, analyses the forming law of ANPS in Liaoxi arid area, and puts the emphasis on the interaction among raining-surface runoff-soil, and the interaction between soil water and ground water. The research result is helpful for preventing non-point sources pollution.