Abstract: The coupling effect of water and nitrogen(N) of cotton under plastic mulching by drip irrigation was studied in Southern Xinjiang.The results showed that there existed a significant coupling effect between water and N.Water stress greatly inhibited N fertilizer effect,while there was a significant difference between cotton yields of treatment with different N application rate under appropriate irrigation.N fertilizer also promoted water effect;however,water is the main effect factor on cotton yield under plastic mulching by drip irrigation in Southern Xinjiang.Based on the results obtained in this experiment,the optimum combination of water and N was: the irrigation 505 mm and N 225 kg per hectare and the canopy indicators of high-yield cotton were: stem height 66.9 cm,leaf emergence 15.6,fruit branches 13.1 and number of bolls each plant 8.9,respectively.Water stress and N deficiency shorten development period of cotton,which maybe the main reason contributed to the yield difference between different treatments.