According to the characters of runoff and fertilization, the runoff process in the farmland was divided into paddy field fertilization period, paddy field growth period and dry land period. Based on the fieldwork in Shanghai area, the regress model of nonpoint source pollution in these three types of farming period were developed. There is difference between the simulated and actual precipitations situations. Nitrogen concentration during the simulated precipitations varied between 28 mg#8226;L-1 and 45 mg#8226;L-1, with the precipitation density at 2 mm#8226;min-1, and the nitrogen concentration during the actual precipitation varied between 0.2 mg#8226;L-1 and 4.0 mg#8226;L-1, with the precipitation density varing between 0.037 6 mm#8226;min-1 and 0.075 1 mm#8226;min-1. Based on the long-term precipitation data and nitrogen loss in the groundwater, by the modified SCS method the annual nitrogen load in the farmland was calculated, and the value is 26.5 kg#8226;ha-1.