In the course of dehydration, the gas exchange and chlorophyll (Chl) fluorescence were measured under irradiance of 800 μmol m-2 s-1 in detached apple leaves, and the production of active oxygen species (AOS), hydrogen peroxide (H2O2), superoxide (O2-), hydroxyl radical (-OH), and singlet oxygen (1O2), were determined. Leaf net photosynthetic rate (PN) was limited by stomatal and non-stomatal factors at slight (2-3 h dehydration) and moderate (4-5 h dehydration) water deficiency, respectively. Photoinhibition occurred after 3-h dehydration, which was defined by the decrease of photosystem 2 (PS2) non-cyclic electron transport (P-rate). After 2-h dehydration, an obvious rise in H2O2 production was found as a result of photorespiration rise. If photorespiration was inhibited by sodium bisulfite (NaHSO3), the rate of post-irradiation transient increase in Chl fluorescence (Rfp) was enhanced in parallel with a slight decline in P-rate and with an increase in Mehler reaction. At 3-h dehydration, leaf P-rate decrease could be blocked by glycine (Gly) or methyl viologen (MV) pre-treatment, and MV was more effective than Gly at moderate drought time. AOS (H2O2 and O2-), prior to photoinhibition produced from photorespiration and Mehler reaction in detached apple leaves at slight water deficiency, were important in dissipating photon energy which was excess to the demand of CO2 assimilation. So photoinhibition could be effectively prevented by the way of AOS production. and H. S. Jia, Y. Q. Han, D. Q. Li.