Water and fertilizer application are the yield improvement approaches in cotton production, especially in arid regions. We hypothesized that irrigation and phosphorus fertilization regimes would enhance cotton leaf photosynthesis capacity. We examined two irrigation levels, i.e., normal irrigation (W1: 3,750 m 3 hm-2) and limited irrigation (W2: 2,500 m 3 hm-2), combined with no phosphorus (P0) and P application (P1: applying P2O5 with the ratio of the fertilizer to dry soil being 0.15 g kg-1). Leaf area (LA), net photosynthetic rate (PN), transpiration rate (E), and quantum efficiency of PSII (YII) of cotton under P1 were higher than that under P0. Under P1 conditions, the increasing rate of fruiting branches LA with W2 was 12.6% higher than that of W1 after 69 d (DAE). PN and E were 9.8 and 6.3% higher, respectively; YII was 38.0% higher at 69 DAE. The total and reproductive organs dry mass (DM) were in the following pattern: W1P1 > W1P0 > W2P1 > W2P0. After 69 DAE, there was a highly significant correlation between the fruit-branch LA and YII, between the fruitbranch LA and PN, and between PN and reproductive organs DM and total DM. Therefore, application of P can enlarge leaf area and slow down chlorophyll degradation, thus promoting accumulation of photosynthetic products in reproductive organs, especially under drought conditions.