Abstract: The precipitation is not stable in dryland farming area, it is also difficult to support water according to crop demand in irrigation area, water and fertilizer can not cooperation well according to people expect. In order to develop the effects of water and nutrient fully, it must to be done firstly to decide the key stage and compensatory effect of water and nutrient cooperation, so that it fits to unstable precipitation. Therefore, it is very important to study the condition and mechanism of water and nutrient cooperation, in order to develop the water and nutrient interaction, to improve the use efficiency of water and fertilizer. The pot experiment was conducted to study the effects of nitrogen application rate and stage on summer maize water use efficiency under different status of soil moisture by using the quadratic general spinning design in this paper. The results showed that the effect of nitrogen fertilizer on WUE is more great than that of soil moisture,but the N application stage for WUE is different between WUE based to grain yield and biological yield. Effects of N application in seedling stage and milking stage on WUE based to grain yield are more great than in other stage,while that of N application in seedling stage and jointing stage on WUE based to biological yield are more great than that in other stage. From the single factor effect,we found that the best N application rate and soil moisture are N 0.2g /kg soil and 21% respectively. Not the more N application rate and soil moisture, the more WUE. The key stage for water and nitrogen interaction is jointing stage. The threshold value existed in water-nitrogen interaction.If the N application rate and soil moisture are below the threshold value the interaction effect of N and soil moisture on WUE will be unremarkable, Otherwise that will be remarkable.