Based on a field experiment of nitrogen fertilizer application in cotton fields under drip irrigation in South Xinjiang, effects of nitrogen application on dynamics of soil Nmin during cotton growth season and soil NO₃^--N residue after harvest of the crop were studied. Results showed that with the nitrogen application rate ranging between N 225~337.5 kg/hm², soil Nmin (NO₃^--N mainly) accumulated mainly in the 0~40 cm soil layer after the flowering stage and the amount of NO₃^--N residue in the plow layer after the crop was harvested varied with application rate. Based on soil NO₃^--N residues and cotton yields of various treatments, an optimum range of nitrogen application rate is determined to be in the range from N 180 to 225 kg/hm² for cotton under drip irrigation to gain high yield while leave less NO₃^--N residue in the soil in South Xinjiang.