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 (W-1: 3,750 m(3) hm(-2)) and limited irrigation (W-2: 2,500 m(3) hm(-2)), combined with no phosphorus (P-0) and P application (P-1: applying P2O5 with the ratio of the fertilizer to dry soil being 0.15 g kg(-1)). Leaf area (LA), net photosynthetic rate (P-N), transpiration rate (E), and quantum efficiency of PSII (Y-II) of cotton under P-1 were higher than that under P-0. Under P-1 conditions, the increasing rate of fruiting branches LA with W-2 was 12.6% higher than that of W-1 after 69 d (DAE). P-N and E were 9.8 and 6.3% higher, respectively; Y-II 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 Y-II, between the fruit-branch LA and P-N, and between P-N 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.