Water and nitrogen(N)are two important factors impacting crops growth,it is necessary to pay much attention to the synergistic regulation effects of water and nitrogen on crops in arid areas,especially in northwest China.To address the coupling effects of water & N interaction on yield and photosynthetic parameters of cotton,a field experiment was carried out to study the growth characteristics in terms of belowground(root)and aboveground(shoot)parts in cotton growing stages and the synergetic effects of water and N regulation on cotton yield components.In this study,three irrigation quotas(ie3 300,3 900 and 4 500 m3·hm-2)and four N rates(ie150,225,300 and 375 kg·hm-2)were set up in a completely randomized block design with three replicates.Results showed that the treatment with water and nitrogen amounts at 3 900 m3·hm-2 and 300 kg·hm-2 promoted photosynthetic parameters（Pn,Tr）and SPAD value and leaf WUE of cotton leaf in different stages.Dry weight of cotton root showed a parabolic curve with nitrogen amounts increasing,while shoot parts increased gradually with nitrogen application amounts increasing.Dry weight of cotton root under different irrigation amounts reached the maximum in each growing stage at N 300 kg·hm-2,but the root/shoot ratio of cotton present declined gradually,also the ratio decreased with irrigation amounts increased.Deficient irrigation(W3 300 treatment)was benefit to increase the ratio of root to shoot,but it led to a significant decline in photosynthetic parameters（Pn,Tr）and SPAD values and leaf WUE.In this experiment,it generated a pleasant cotton height at 79 cm and an improvement in numbers of available bolls and single boll weight when irrigation quota at 3 900 m3·hm-2 and N rate at 300 kg·hm-2,available bolls increased 1.0 and 0.4 per plant,single boll weight increased 0.16 and 0.09 g,yield was increased by 8.35% and 4.62%,and the lint percentage was increased by 1.4% and 0.7%,compared with those of the treatments of W3 300 and W4 500.Therefore,the suitable ratio of root to shoot of cotton was about 0.111.