Abstract: As the largest bread basket in Zhejiang Province, Hangjiahu area is facing more and more serious water pollution, while the N loss loading by runoff from the paddy field during its submersed period is the main cause of the pollution. Through field experiment and fixed spot observation, the model of precipitation-runoff in Yangtze delta was testified, and the results showed that the precipitation-runoff model from HE Baogen was basically accorded with the fact after considering the impact of field overflow mouth, and the error was between -19.9% and + 18.0%. The model of N concentration with precipitation-runoff in paddy field during submersed period was brought forward, with the R value being 0.948. These two models consisted of the model of N loss loading by runoff from paddy field during submersed period. Based on this model as well as the past 30 years data of fertilization and precipitation, 1∶250 000 topography map, land use map, and water system map, the N loss loading and its distribution were estimated by using GIS method, and the results showed that the N loss loading was different from place to place, with an average of 35.26 kg N·hm^-2, and accounting for 12.69% of the applied N. The N loss loading in Anji and Yuhang with obviously more precipitation was higher than that in other places, while Haining also had a serious N loss problem because of the huge amount of applied N.