Abstract: The different physiological responses and their mechanisms in the leaves of Reaumuria soongorica from two altitudes when under osmotic stress were studied. When the leaves of R. soongorica were seriously stressed, the leaves from the branches picked at the foot of the Jiuzhoutai mountain had a better water condition, higher contents of proline and soluble sugar, lower malondialdehyde (MDA) content and reactive oxygen level. The materials living at the foot of the mountain were better in hardness than those living on the top of the mountain. The interrelationship analysis among the above indexes showed that there were significantly negative relationships between the relative water content, water potential, the ratio of free water content with bound water content and MDA content, and O₂^-· production rate, and there was a significantly positive relationship between superoxide dismutase (SOD) activities and O₂^-· production rate. With an increase of osmotic stress, the material living at the bottom had better osmoregulation and antioxidative ability, and even lower level of lipid-oxidation. The main reason for the above differences may be the water content: the material living at the bottom of the mountain had higher levels of bound water, and better water retaining ability under deep osmotic stress. The relatively lower water content and water potential of the material living at the bottom could be one of the important reasons for the better osmo-regulation ability to deal with osmotic stress.