Dolichomitriopsis diversiformis, an endangered moss species which is unique to East Asia, is now distributed in a very narrow area. From our early field investigation of its habitat, we found that the species is restricted by moisture content of atmosphere and growth matrix. In order to disclose the endangered mechanism of D. diversiformis under water stress, and to elucidate eco-physiological differences of bryophytes in similar humidity habitat, Plagiomnium acutum, a widespread species was selected to compare with D. diversiformis. Therefore, their photosynthetic chlorophyll characteristics of phtosystem Ⅱ (PSⅡ), such as electron transport rate (ETR)- photosynthetic available radiation (PAR) curves, maximal photochemical efficiency(Fv/Fm), photosystem quantum yielding (Y(Ⅱ)), photochemical qunching (qP) and non-photochemical qunching (NPQ) were detected by PAM 2500 and IMAGING PAM. Furthermore, metabolism of reactive oxygen species (superoxide anion and hydrogen peroxide), activities of antioxidative enzymes (superoxide dismutase (SOD), catalase (CAT) and ascorbate peroxidase (APX)) and content of ascorbic acid (ASA) in the two moss species during desiccation and rehydration were determined in our studies. Results of ETR-PAR curve indicated that desiccation reduced the inhibitory light density of both species; but when water content recovered, the inhibitory light density restored to normal status. Compared with P. acutum, ETR of D. diversiformis was blocked completely by very weak light density after 60 min dehydration and recovered much more slowly in rehydration. And the results of other PSⅡ chlorophyll fluorescence characters also gave similar evidences. Fv/Fm and YⅡ of both bryophytes were decreased under desiccation and then increased in rehydraion, but the value of D. diversiformis declined more quickly during desiccation and recovered more slowly in rehydration. That is to say, photosynthetic electron transport of PSⅡ in D. diversiformis can be not only reduced by water loss but also destroyed completely by deep desiccation. Meanwhile, variation of qP and NPQ of both species manifested an interesting occurance. qP of D. diversiformis declined more quickly than P. acutum in desiccation and returned to normal condition much more quickly than that of P. acutum. In other words, photosynthetic reaction which is represented by photochemical qunching is protected primarily in D. diversiformis. But photochemical quenching restoration would bring out more production of reactive oxygen species. Otherwise, though NPQ of both species increased in desiccation and decrease in rehydration, yet the absolute value of D. diversiformis maintained a relative low level. These results suggested that the photo-protective mechanism supported by xanthophyll cycle in D. diversiformis should be weaker than in P. acutum. All these results indicate that PSⅡ of D. diversiformis, especially reaction central pigment (P680) is more sensitive to dehydration than P. acutum. Results of reactive oxygen species metabolism showed that absolute value of superoxide anion production and hydrogen peroxide content in D. diversiformis were significantly higher than in P. acutum. Furthermore, though anti-oxidative enzymes, such as SOD, CAT, APX also increased similar of reactive oxygen species under desiccation, yet their activities in D. diversiformis decreased more quickly in rehydration. In conclusion, the sensitivity of photosystem Ⅱ and the low efficiency of antioxidative system in rehydration are probably two major eco-physiological restricting factors of D. diversiformis narrow distribution.