In this study, the polders in the upper reaches of Taihu Lake were taken as the study area, and the dynamic process of N and P loss in polders were simulated using Nitrogen Dynamic model for lowland Polder systems (NDP) and Phosphorus Dynamic model for lowland Polder systems (PDP). Based on the mechanism of N and P loss in polders, the management scenarios were quantitatively simulated and the benefits of N and P pollution reduction were analyzed. The results showed that:1)The annual average loss intensities of N and P in the polders were 40 and 1.75 kg/(hm²·a), respectively, which were higher than the average loss level of Taihu Lake Basin; 2)When the water quality in the external rivers of polders was controlled as Grade IV, the loss intensities of N and P were 37.54 and 1.58 kg/(hm²·a) respectively, the amount of N and P intercepted in the polder increased by 4.9% and 9.9% respectively, and the number of polders with strong loss level decreased by 26.2% and 63.8% respectively; 3)If 5% of paddy fields in polders were converted into water bodies, the loss intensities of N and P were 36.73 and 1.72 kg/(hm²·a) respectively, the amount of N and P intercepted in polders increased by 6.6% and 1.7% respectively, and the number of polders with strong loss level decreased by 46.2% and 22.5% respectively; 4)The loss mechanisms of N and P in the polders were obviously different. The water quality in the external rivers of polders had a strong influence on the loss of P in the polders, and the water bodies and ponds in the polders had a greater advantage for N retention and purification.