Analysis was conducted of quantitative characteristics and budget of N in soil under the winter-wheat/summer-maize rotation system in the North China Plain, and optimal N potential evaluated. Results show that the N input was mainly through chemical fertilizer, organic manure, rainfall, irrigation, biological fixation and seeds in the system, contributing 545 kg hm^-2 yr^-1, 68 kg hm^-2 yr^-1, 21 kg hm^-2 yr^-1, 15 kg hm^-2 yr^-1, and 5 kg hm^-2 yr^-1, respectively, totaling N input was 669 kg hm^-2 yr^-1, while the N output with harvested crops was only 311 kg hm^-2 yr^-1. In addition, the N losses through ammonium volatilization, denitrification and leaching was 120, 16 and 136 kg hm^-2 yr^-1, respectively, making the total N output up to 583 kg hm^-2 yr^-1with a surplus of 86 kg hm^-2 yr^-1 N. Intensifying N management to improve its use efficiency in combination with intensifying application of organic manure is the long-term task in N management in the North China Plain.