Abstract: As the main factors that influence crop yield and quality, drought has been the focus of agro-ecology for decades. To determine the physiological response of plants to fast water stress, root drought was simulated along with photosynthetic control of leaf surface air humidity in tomato. Polyethylene glycol (PEG-6000) solution was used in the drought study. Changes in tomato leaf water potential, stomatal conductance, osmotic adjustment substances and other physiological parameters were monitored. The results showed that the net photosynthetic rate (P_n) of tomato leaves under water stress was lower than that of the control when stomatal conductance (G_s) was less than 120 mmol·m^-2·s^-1 and vice versa. A significant negative exponentiation correlation was noted between water vapor pressure deficit (VPD) and stomatal conductance (G_s) in tomato. VPD was significantly positively correlated with G_s (R₂ ≥ 0.892). Under specific air humidity condition, maximum G_s under water stress treatment was only 43%~51% that of the control. Soluble sugar content of tomato leaves increased initially and then decreased under the rapid water stress condition. Soluble sugar content increased significantly by 61.4%~195.8% and 56.0%~167.5% over the control. Tomato leaf Gs exhibited a threshold response to P_n, and which threshold value was 120 mmol·m^-2·s^-1. Soluble sugar was suitable for supporting physiology process such as cell turgor pressure, stomatal opening and photosynthesis. The fluctuation in stomata opening was the mode of adaptation of tomato plant to water stress. Regulation of stomata opening was a critical form of response of tomato plant to drought stress.