Abstract: Desert algae have a cosmopolitan distribution in arid and semi-arid regions, and are frequently subjected toabsence of water source for survival demands. They are poikilohydric organisms whose lifestyle in many cases consistsof alternating periods of drought. A green algae Palmellococcus sp., isolated from desert algal crusts, is apparently subjectedto such extreme transitions between dehydration and rehydration. In this study, the effect of dehydration andsubsequent rehydration on PSⅡ activity (Fv/Fm), MDA contents and intracellular production of water-soluble substancesas well as antioxidant enzyme activities were investigated for Palmellococcus sp. in laboratory. Concerning the experiments,three drought treatments at different relative humidity (RH) were designed as follows: RH= 0% (exceedinglydroughty), RH=43% (moderately droughty) and RH=98% (fully hydrated, as control). Subsequently, rehydration wereconducted by using of BBM medium, extracellular polysaccharides (EPS, 0.2 g/L), sucrose solution (0.2 g/L), prolinesolution (0.2 g/L) and distilled water as well as 2.5mM CMP and 50μM DCMU. The results showed that drought resultedin a rapid decrease in PSⅡ activity of Palmellococcus sp. and increased antioxidant enzyme activities of catalase(CAT, EC 1.11.1.6) and superoxide dismutase (SOD, EC 1.15.1.1) when compared to the control treatment. It also indicatedthat intracellular production of water-soluble substances such as saccharides and proteins were comparativelygreater in the dehydrated cells than the control cells, and simultaneously drought caused a significantly deleterious effecton membrane lipid superoxide as assessed by measurement of MDA content. In addition, we found that hydrationinduced an apparent increase in PSⅡ activity of Palmellococcus sp., and the roles of exterior exopolysaccharide (EPS)and sucrose in photosynthetic recovery were more effective than other rehydration treatments. Research results mayprovide us a significant finding for better understanding the tolerance mechanism of desert green algae to drought environment.