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The fundamental data of this research, obtained by field survey, positioning monitor and laboratory test, was used by simulation model. We employed the Soil and Water Assessment Tool （SWAT）, used widely over the world, to investigate pollutant loading from farmland. The usability of SWAT model was also analyzed. After all, we applied SWAT model to predict the impact of alternative management practices on water quality and quantity of field drainage. From this research, we concluded that:（1） During the vegetation period, from April to October, the highest concentration of NH4-N was 4mg L-1, and the average concentration was 1 mg L-1, above the allowable value of classⅢ, specified by“National Environmental Protection Standard”, and belongs to medium pollution . The average concentration of TN was 4.6 mg L-1, far exceed the allowable value of class V, and belongs to heavy pollution.（2） Lots of fundamental data were gained by field survey and laboratory test. ArcGIS was used to build the spatial database. SWAT was modified to adapt local conditions, and then we develop a field drainage pollution assessment model. Measured data was used to sensibility analysis of parameters, model calibration and validation. The results showed that modified SWAT was quite satisfactory （NS > 0.6, R2 > 0.7）.（3） The model was employed to investigate the source, spatial and temporal distribution of field drainage pollution load, the simulation results indicated that the nitrogen and phosphorus load mostly come from cultivated area. TN and TP loads of it accounted for 98.99% and 99.23% of the total loads, respectively. And most TN and TP loads of cultivated area caused by paddy field, constituted separately 77.34% and 75.65% of the total loads. The spatial distribution of nitrogen and phosphorus loads were in accordance with land use, mainly distributed in cultivated area. And the nitrogen and phosphorus were almost discharged during irrigation and fertilization season, from May to August.（4） The model was applied to simulate the impact of management practices on nitrogen load and yield of paddy field. The result showed that, TN load and yield of paddy field were decreased when N fertilizer decreased, but the kind of curve were different, the formerly was logarithm, while the latter was exponent. Compared to current situation, a 20% reduction in N fertilizer was beneficial, because the TN load decreased by 24.3%, while yield decreased only 4.4%, above 9000 kg hm-2. The pattern of N fertilizer had a marked effect on TN load. TN load could be reduced by increasing base N fertilizer, decreasing after N fertilizer, while had no effect on yield.
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