Abstract: 【Objectives】 Nitrogen (N) and phosphorus (P),two essential nutrients for plant growth, play important roles in increasing crop yields and improving the quality of agricultural products. However, unreasonable applications of N and P fertilizers cause surplus of nutrients in soil, and their losses alone with rainfall or irrigation would ultimately lead to the decrease of environmental water quality. In this study, a long-term field lysimeter study was carried out to investigate the possible effect of combined application of organic and inorganic fertilizers in alleviating the leaching of N and P from the soil, to provide scientific support for protection of groundwater quality.【Methods】 The leaching of N and P had been monitored in the National Soil Fertility Long-term Experimental Station in Changping City, Beijing for 5 consecutive years. The treatments include: control (no application of N fertilizer, PK), only chemical fertilizer (NPK), only organic fertilizer with the same N application rate as NPK treatment (SW), and combined organic and chemical fertilizer (replacing the 50% dosage of N fertilizer with pig manure). During the monitoring period, the leachate was collected at a soil depth of 120 cm and the concentrations of N and P in the leachate were measured to study influences of combined application of the organic and chemical fertilizers on maize yield and N and P leaching, under the same rate of N application to the field.【Results】 1)The combined application of 50% N in pig manure and 50% N in chemical fertilizer (SNP) ensures a high level of maize yield,the 5-year’s average yield is 10.3% higher than NPK treatment. 2)Compared with the NPK treatment, the total N (TN) leaching was significantly reduced by 71.4% through the combined application of organic and inorganic fertilizers. The average concentration of NO-3-N in leachate under the NPK treatment (54.93 mg/L) is significantly higher than that under the SNP treatment (13.47 mg/L). 3)Under the equal N application rate, the amount of P applied in pig manure treatment (SW) is much higher than that in the NPK treatment. As a result, the total P (TP) leaching under the SW treatment is 0.6 times higher than that under the NPK treatment. The average concentrations of TP in leachate of the SW and NPK treatments are 0.09 mg/L and 0.066 mg/L, respectively. The amount of NO-3-N accounts for 80% of TN in leachate, and the dissolved form of P accounts for about 70% of TP. 4)The average concentration of NO-3-N in leachate from the NPK treatment exceeds the threshold value for Class III of groundwater quality (GB/T 14848-9). Moreover, the concentration of TP in leachate from SW (0.09 mg/L) is higher than the threshold of TP (0.02 mg/L) for water eutrophication. 【Conclusions】 The amount of NO-3-N accounts for 80% of total N in leachate, and total dissolved P accounts for about 70% of TP. The 5 years’ average maize yield in the treatment of combined application of the organic and chemical fertilizers proves that this treatment can significantly increase the yield, at the same time, could basically guarantee the concentration of NO-3-N in leachate at a level lower than the threshold value for Class III of groundwater quality (GB/T 14848-9).