Abstract: To investigate the dynamic changes and response mechanisms of gas exchange parameters and fluorescence indices of the subtending leaf to cotton boll under soil progressive drought stress, pot experiments of the hybrid cotton No.3 were conducted with soil relative water content (SRWC) (75±5) % as control group, SRWC (60±5) % and SRWC (45±5) % as experimental groups dealt with progressive drought for 50 days. Results showed that, the net photosynthetic rate (P_n), stomatal conductance (g_s) and leaf intercellular CO₂ concentration (C_i) decreased while L_s increased under SRWC (60±5) %  for 0-21 days. Furthermore, there was no significant change in chlorophyll fluorescence indices. This indicated that stomatal limitation was the main reason for the reduction of photosynthesis of cotton. In addition, when drought for 21-49 days under SRWC (60±5)%, P_n kept decreasing, while C_i began to increase and L_s began to decrease. Potential photochemical efficiency (F_v/F_m), quantum yield of photo system II (Φ_PSII) and photochemical quenching coefficient (q_P) reduced significantly, but nonphotochemical quenching coefficient (NPQ) first rose then decreased. Thus, nonstomatal limitation was the main reason why the photosynthesis of cotton reduced. Photosynthetic organization and photosynthetic enzyme system were destroyed, boll setting intensity reduced and the number of boll and yield reduced significantly. Drought for 0-14 days under SRWC (45±5) % treatment led to sharp decrease in P_n, g_s and C_i, whereas L_s obviously increased. There was no significant change in F_v/F_m, Φ_PSII, q_P, indicating stomatal limitation was the main reason why the photosynthesis of cotton reduced. P_n decreased slowly, while C_i began to rise and L_s began to decline under SRWC (45±5) % treatment for 14-49 days. F_v/F_m, Φ_PSII, q_P  decreased while NPQ rose first then declined, which indicated that non-stomatal limitation worked to reduce the cotton photosynthetic performance. The boll setting intensity reduced significantly and the number of boll and yield declined. The critical stress time of cotton growth in current study was 21 and 14 days respectively under SRWC (60±5) % and SRWC (45±5) % treatments during the flowering and boll-forming stage.