A field case study was conducted to elucidate the dynamic characteristics of N leaching in paddy fields during the ricecropping season. The results showed that, nitrate (NO-3-N) is the dominant species in natural leaching water of paddy soil. High NO-3-N concentration in paddy field and fertilizer application are the main reasons of the heavy N leaching loss at the first few days of immerging. However, the leaching loss amount is weak in later paddy planting because the soil nitrification is inhibited under the condition of immerging. The leaching amount of N is affected by the concentration of N and leaching rate, while the leaching rate is controlled by perviousness of the plough pan and the hydrographic conditions. Under the conditions of an application rate of 250 kg/hm2 and a percolation of 200 mm, the N leaching load of 6.44 kg/hm2 and are 2.58% of total fertilizer application amount are estimated for the singlerice cropping paddy.