Abstract: The influences of water stress on gas exchange and chlorophyll fluorescence parameters of Ammopiptanthus mongolicus were studied using Li-6400 portable photosynthesis system. The results showed that the relationship between net photosynthesis rate, transpiration rate, stomatal conductance of A. mongolicus and water potential were stimulated by y= a+b(1原e-kx), and the relation between chlorophyll fluorescence parameters and water potential were y=ae-bx^k. The net photosynthesis rate, transpiration rate, stomatal conductance and chlorophyll fluorescence parameters of A. mongolicus decreased under water stress. The initial water stress potential of net photosynthesis rate, transpiration rate and stomatal conductance were less than those of chlorophyll fluorescence parameters, and the photosynthetic water potential compensation point, water potential at transpiration rate critical point and stomatal conductance critical point were similar to those of chlorophyll fluorescence parameters(NPQ, qP and ETR), but that were much less than initial water stress potential of Fv/Fm. The water potential at stomatal conductance critical point could be used as the limit of estimating the photosynthetic stomatal and nonstomatal regulation, and the initial water stress potential of Fv/Fm was used as the critical value of plant water stress. When the water potential was more than the water potential at stomatal conductance critical point, it belonged to the water potential of stomatal regulation, however, when the water potential was less than the water potential at stomatal conductance critical point, it belonged to the water potential of nonstomatal regulation. When the water potential was more than the initial water stress potential of Fv/Fm, it belonged to the mild or moderate water stress, if it was less than the initial water stress potential of Fv/Fm, it belonged to heavy water stress. The result showed that the water potential at stomatal conductance critical point of A . mongolicus was between -3.81 and -5.26 MPa, and the critical point of heavy water stress was between -8.05 and -14.53 MPa, and they had the seasonal difference.