A field plot experiment was conducted and showed that water irrigation rate and nitrogen application rate were main factors affecting leaching and transformation of urea in fluvo-aquic soil. It was easily transformed into NH₄⁺-N and NO₃-N when urea was applied into the soil at turning-green stage of winter wheat treated with irrigation. NH₄⁺-N accumulated only in 0~40 cm layer and did not go down with leaching into deeper layers. However, NO₃-N concentration varied sharply in the soil profile. When the irrigation rates were the same, the more N was applied, the deeper NO₃-N went down in the profile, and when the N application rates were the same, the higher the irrigation rate was, the deeper NO₃-N was leached down in the profile. Even though the irrigation and nitrogen application exceeded the usual rate (750 m³/hm² and N 180 kg/hm², respectively), NO₃-N accumulated in the soil layer 0~130 cm, and never went down beyond 130 cm in all treatments when the crop was ready for harvest. So during the wheat growing season N application will not contaminated the underground water.