A one-dimension model of nitrogen cycling was described in this paper. The model constituted of three modules:soil-water-movement module, nitrogen-transport module and rice-growth module. In simulating soil water movement, potential saturated and unsaturated flow in soil were considered; In simulating nitrogen transport, NO₃^-, NH₄⁺, and urea were taken as soil solutes and their vertical movement were simulated by convection-dispersion equation; In simulating nitrogen transformation, urea hydrolysis, mineralization, ammonia volatilization, nitrification, denitrification and ammonium adsorption-desorption were considered together; In simulating crop growth, ORYZA rice-growth model was selected and some parameters were modified by field experiment. The model was validated by a 3-year field experiment, and simulated results of mineral nitrogen in soil and rice-growth were proved well. Comparison analysis showed there were significant correlation between simulated and measured ammonium nitrogen concentrations, as well as dry biomass of rice different organs.