Reaumuria soongorica(pall.)Maxim,a perennial desert semi-shrub,is widely found in semi-arid areas of China.It shows strong tolerance to drought,high temperature and intense radiation in its natural habitat.In the present study,photoprotective mechanism of R.soongorica was investigated by analyzing diurnal variations of gas exchange and chlorophyll fluorecence parameters during progressive soil drying.The results show that leaf water potential of R.soongorica decreased when the soil water content dropped.Diurnal patterns of net photosynthetic rate(Pn)changed from "two peaks" to "one peak" under drought stress,and the leaf water use efficiency(WUE)increased under moderate drought and declined under severe drought.Pn,the primary maximum photochemical efficiency of PSII(Fv/Fm)and the quantum efficiency of non-cyclic electron transport of PSII(φPSII)decreased obviously at noon,and showed a photoinhibition phenomenon.But,the non-photochemical quenching of fluorescence(NPQ)soon reached its maximum in the day and then remained almost at the high level until 17:00,indicating that the xanthophylls cycle-dependent thermal energy dissipation played an important role.Diurnal variations of the original chlorophyll fluorescence(Fo)increased at first and then decreased.The increased value of Fo under drought stress indicates that there was a reversible inactivalion of PSII reaction center.These results indicate that the photoprotective mechanism in R.soongorica was the photoinhibition by using both the xanthophylls cycle-dependent thermal energy dissipation and the reversible inactivation of PSII reaction center under drought stress.