We evaluated the change mechanisms of Pistacia weinmannifolia leaves fast chlorophyll under water stress condition,which is typical species in dry and hot valley.The present study was designed to measure fast chlorophyll fluorescence under 5 relative soil water contents 50%(C₁),30%(C₂),20%(C₃),10%(C₄)and 5%(C₅).The results showed:(1)under the water stress condition,oxygen evolving complex subject of P.weinmannifolia is damaged hardly,and a large number of photosynthetic electron transport chain Q_A^- are accumulated,electron transfer capability and quantum yield were lower.(2)The minimum and maximum points of ABS/CS,TR_o/CS,RC/CS,DI_o/CS and ABS/RC,ET_o/RC,DI_o/RC,M_o arrived under C₃ level,the minimum and maximum points of V_j and Ψ_o,ΨE_o,ET_o/CS arrived at C₂ level,ET_o/RC are increased with the relative soil moisture content lower.(3)P.weinmannifolia PI_ABS (photosynthetic performance index) is more sensitive than F_v/F_m (φp_o).From this reseach,it can be seen that P.weinmannifolia has launched a corresponding self-defense mechanism,plastoquinone Q_A are slowing the reduced rate,electronic hindered in the transfer process under the C₂ level.The antenna system of P.weinmannifolia has been damaged,which leading to reduced absorption of light energy and dissipation of excessive excitation energy in time under the C₃ level,and reduce the OEC damage caused by the reaction center,only be lifted at C₄,and improve the P.weinmannifolia photosynthetic efficiency.Water stress will impact the P.weinmannifolia physiological,which will grow health under the relative soil water content keep 10%,the P.weinmannifolia can adapt the drought conditions.