Abstract: Optimal farmland management measures are currently the main means of controlling surface source pollution in the Erhai Lake basin. This study aims to assess the effectiveness of various management strategies to mitigate surface source pollution in the basin and to identify the best practices for farmland management. Using a rate-determined and validated SWAT model, we simulated the effect of reducing nitrogen and phosphorus losses under 15 different scenarios, which included reducing the amount of chemical fertilizer application, altering cropping patterns, establishing vegetation buffer zones, planting grasses in river channel, and implementing comprehensive management measures. The results showed that the calibrated SWAT model demonstrated satisfactory performance, with a coefficient of determination (R²) of at least 0.67 (exceeding the threshold of 0.60) and a Nash efficiency coefficient (Ens) of at least 0.56 (surpassing the required 0.50), confirming its suitability for simulating effective management measures in the Erhai Lake basin. The analysis revealed that reducing fertilizer application by 10%, 20%, and 50% could lead to nitrogen reductions of 9.1% to 16.6% and phosphorus reductions of 0.9% to 4.4%. Among the various tillage practices examined, deep tillage proved most effective in controlling nitrogen and phosphorus losses, achieving reductions of 13.8% and 14.1%, respectively. The establishment of vegetation buffer zones resulted in nitrogen loss reductions ranging from 9.3% to 22.5% and phosphorus loss reductions from 10.4% to 24.6%. The implementation of grass-vegetated river channels led to nitrogen reductions of 21.3% to 39.6% and phosphorus reductions of 24.9% to 45.2%. Compared with individual measures, comprehensive measures demonstrated significantly higher efficiencies in reducing nitrogen and phosphorus levels. For example, the combination of a 20% reduction in fertilizer application with deep ploughing achieved nitrogen reductions of 27.4%, which increased to 43.7% with the addition of a 5m vegetation buffer zone, and 41.6% with a 20m grassed river channel. The corresponding phosphorus reduction efficiencies were 19.2%, 39.7%, and 61.1%, respectively. In conclusion, the integrated management measures substantially reduced surface source pollution loads in the watershed compared to individual strategies. This study not only quantifies the impacts of various farmland management practices on surface pollution in the Erhai Lake basin but also provides a theoretical foundation for effective prevention and control measures against surface pollution.