Abstract: Impacts of different substrate water potentials (SWP) on leaf gas exchange and chlorophyll fluorescence parameters of greenhouse cucumber during its postflowering growth stage were analyzed in this study. The results demonstrated that -10 and -30 kPa were the critical values for initiating stomatal and nonstomatal limitation of drought stress, respectively. During the stage of no drought stress (-10 kPa<SWP≤0 kPa), gas exchange parameters and chlorophyll fluorescence parameters were not different significantly  among treatments. During the stage of stomatal limitation of drought stress (-30 kPa<SWP≤-10 kPa), with the decrease of SWP, the stomatal conductance (g_s), intercellular carbon dioxide concentration (C_i), net photosynthetic rate (P_n), apparent quantum efficiency (ε), transpiration rate (T_r), carboxylation efficiency (CE), maximum Rubisco
limited rate of carboxylation (V_{c max}), maximum rate of electron transport (J_max), rate of triosephosphate utilization (V_TPU), maximum and actual quantum efficiency of PSII (Φ_PSⅡ and F_v/F_m) and photochemical quenching (q_P) decreased, but the light compensation point (LCP), dark respiration rate (R_d), carbon dioxide compensation point (CCP), stomatal limitation value (L_s), instantaneous water use efficiency (WUE_i) and non-photochemical quenching (q_N) increased. In this stage, gas exchange parameters changed faster than chlorophyll fluorescence parameters and differed significantly among treatments. During the stage of nonstomatal limitation of drought stress (-45 kPa≤SWP≤-30 kPa), with the decrease of SWP, light saturation point (LSP), R_d, CE, V_{c max}, V_TPU, L_s, WUE_i, Φ_PSII, F_v/F_m and q_P decreased, while CCP, C_i and q_N increased. In this stage, chlorophyll fluorescence parameters changed faster than gas exchange parameters and differed significantly among treatments. In production of greenhouse cucumber, -10 and -5 kPa should be the lower and upper limit value of irrigation, respectively. The stomatal limitation of drought stress could be relieved by irrigation before SWP decreased to -30 kPa. While, the non-stomatal injury of drought stress would be unrecoverable once SWP decreased to lower than -30 kPa.