AgNPs model is a computer model developed by the Agriculture Bureau of the United States Department of agriculture, Minnesota pollution control bureau and natural resources protection bureau. It is used to simulate soil erosion and nutrient loss in small watershed, and to predict and evaluate agricultural non-point source pollution. It can not only predict the non-point source pollution load, but also analyze the risk and investment / benefit. The model consists of three parts: runoff simulation, the basis of non-point source pollution; sediment simulation, mainly reflecting the migration mechanism; and finally, pollutant simulation.