In order to guide the rational fertilization of paddy fields and prevent non-point source pollution of paddy fields, we studied the effects of different nitrogen fertilizer reduction ratios on the nitrogen and phosphorus loss in the surface water of paddy fields under the milk vetch (Astragalus sinicus L.) -rice rotation system. In 2020, field plot experiment was conducted on Jiande, Zhejiang. Two treatments of winter idle and winter milk vetch were set up, and four nitrogen reduction ratios of 0 (CT0), 10% (CT1), 20% (CT2) and 30% (CT3) on the basis of winter planted milk vetch. There were 5 treatments in total, each treatment was repeated three times. After paddy transplanting and fertilization, surface water samples of paddy field were collected (including continuous sampling within two weeks of fertilization and sampling at intervals of 7, 14, and 28 days after two weeks), and the concentration of nitrogen and phosphorus in the field water was measured. After the rice matured, soil and plant samples were collected and soil physicochemical properties, rice growth properties and yield were determined. The total nitrogen, soluble nitrogen, ammonium nitrogen, total phosphorus and soluble phosphorus in the field surface water of each treatment reached their peaks on the first day after fertilization. The total nitrogen decreased significantly within four days after basal fertilizer application, which was 4.2%~9.1% of the maximum value, and the soluble phosphorus decreased to 4.7%~13.7% of the maximum value within five days of basal fertilizer application. During the sampling period, the average concentration of total nitrogen, soluble nitrogen, total phosphorus and soluble phosphorus in the CK was 48.87, 36.82, 0.82 and 0.64 mg/L, respectively. The average concentration of total nitrogen in CT0, CT1, CT2 and CT3 was 93.9%, 78.1%, 79.7% and 69.7% of CK, respectively. The average concentration of soluble nitrogen was 95.1%, 84.1%, 85.7% and 73.2% of CK, respectively. The average concentration of total phosphorus was 90.9%, 76.9%, 96.2 and 81.3% of CK, respectively. The average concentration of soluble phosphorus was 79.4%, 73.8%, 87.3% and 68.7% of CK, respectively. Compared with CK, CT2 and CT3 significantly increased soil available phosphorus content by 61.7% and 37.0%, respectively. Compared with winter fallow treatment, turning over and pressing milk vetch increased rice plant height by 0.7%~3.5%, effective panicle number by 7.0%~15.2% and rice yield by 0.4%~4.9%. Compared with the treatment of winter fallow, the combination of winter planting of milk vetch and different proportions of nitrogen fertilizer reduction could reduce the risk of nitrogen and phosphorus loss in paddy field surface water, of which 30% nitrogen fertilizer reduction was the best. Milk vetch combined with nitrogen reduction fertilization measures improved content of soil available phosphorus and total nitrogen and rice yield. In these treatments, milk vetch combined with 20% nitrogen reduction fertilization had the best effect. Considering the rice yield, soil fertility and risk of loss of nitrogen and phosphorus in paddy field surface water, the combination of milk vetch and 20% nitrogen fertilizer reduction was a more suitable planting method for this area.