The global environmental change caused by the greenhouse gas emissions led to the increasing changes of temperature and precipitation patterns, and drought and other extreme weather events have occurred frequently. In order to understand and predict the effects of global change on terrestrial ecosystems, it is necessary to obtain the responses of leaf photosynthetic parameters to global change required by process-based model. The responses of photosynthetic parameters of Leymus chinensis to drought and rewatering would be studied in this paper, in order to reveal the mechanisms of the photosynthetic parameters of Leymus chinensis responding to water stress and rewatering, and to develop the model of photosynthesis parameters responding to water and temperature for accurately simulating photosynthesis. The photosynthetic parameters of typical steppe grass Leymus chinensis was studied based on the data from the greenhouse simulating experiment in Institute of Botany, the Chinese Academy of Sciences and the field observation in Inner Mongolia typical steppe ecosystem research station, Institute of Atmospheric Environment, China Meteorological Administration, Shenyang. The photosynthetic parameters Vcmax (the maximum rate of carboxylation), Jmax (the maximum potential rate of electron transport) and TPU (the triose phosphate utilization) were obtained from the ACI curve (the relationship curve of net photosynthetic rate and intercellular CO2 concentration) of Leymus chinensis measured by the portable photosynthesis system (Li-cor, Lincoln, NE, USA). The relationship between the photosynthetic parameters of Leymus chinensis and soil moisture content could be expressed as a parabola curve. The maximum values of Vcmax, Jmax and TPU would appear when the soil moisture contents were 15.56%, 15.89% and 16.23% in greenhouse conditions and 16.89%, 17% and 16.79% in the field, respectively. The influence of rewatering on the photosynthetic parameters of Leymus chinensis depended on the level of water stress prior to it. The photosynthetic parameters of Leymus chinensis under the soil moistures of 18%-19% and 15%-16% could recover from the water stress quickly, even some of them would increase. It implied that slight water stress could improve photosynthesis and photosynthetic parameters. However, the photosynthetic parameters under the soil moistures of 10%-12% and 7%-9% could not recover from the water stress previous. This study also indicated that when the soil moisture is less than 10%-12%, the photosynthetic parameters of Leymus chinensis would not recover to the normal level even if the rewatering happens.