Nansi Lake is the largest lake in Shandong province and the northern area of China, which is an important buffer lake of the South-to-North Water Diversion Project (east route). Eutrophication has become the central problem of water environment in Nansi Lake where the eutrophic state is from mesotrophic state to hypereutrophic state. A field study was conducted to investigate the characteristics of inorganic N loss by leaching and runoff during the maize growing season in the Nansi Lake Basin, and determine fertilizer application strategies to reduce N losses and their potential threats to lake water quality. Under different fertilization patterns (In this experiment, 7 different fertilization treatments were installed, such as follows: (1) CK: no fertilizer was applied to soil; (2) FP: farmer's practice of traditional fertilization; (3) PK: in this treatment, only phosphate and potash fertilizer applied to soil; (4) OPT: nitrogen, phosphate and potash fertilizers were mixed to a balanced nutrition; (5) CRF: in the treatment, controlled-release nitrogen fertilizer replaced common nitrogen fertilizer, and other nutrition was invariable; (6) 80%OPT+M: total amount of nutrition applied to soil was equal to the 4th treatment, but 20% of total inorganic nutrition was replaced by that existed in the organic matter-poultry excrement; (7) OPT+St: a balanced N, P and K nutrition was applied to soil, and total amount was equal to the 4th treatment, meanwhile, wheat straw was returned to cover the soil surface), Collectors for leaching and surface runoff water were installed in-situ in each treatment. Water samples were collected for analysis of different forms of inorganic nitrogen in laboratory. The results indicated a positive correlation between quantity of leachate water, surface runoff water and precipitation. The amount of water was affected by substances such as wheat straw; The Concentration and total amount of NO^-₃-N and NH⁺₄-N losses from runoff were significantly higher than their losses through leaching, and accounts for 2/3 or more of N losses, which was the primary means of N losses in the form of aqueous solutions. N losses through leaching and runoff were mainly NO^-₃-N form (NO^-₃-N runoff loss accounted for 82.9%-90.8% of total, and leachate water loss accounted for 63.5%-72.9% of total N. Concentration of NO^-₃-N in runoff water greatly affected water quality, but it did not constitute a threat to groundwater pollution through leaching; N losses through leaching and runoff were highest in treatment FP (Farmers practice) for the whole maize growing season. To maintain maize grain yield while reducing N pollution of the lake, a balanced application of NPK fertilizer is required, combined with the application of controlled-release nitrogen fertilizer, organic alternative to inorganic and the use of straw incorporated into the soil. Appropriate practices will need to be introduced to the farmland in the Nansi Lake basin.