In order to understand the influence of drought stress on hulunber grassland productivity, the photosynthetic characteristics of Leymus chinensis with four soil moisture grades were measured with Li-6400 Portable Photosynthesis System in hulunber prairie. The results indicated that the diurnal patterns of net photosynthetic rate (Pn) of Leymus chinensis presented two peaks curve under moisture treatments of（ 40±1）%, （20±1）% and （10±1）%. The first peak appeared at 8:00, and the sub\|peak at 16:00, with an obvious mid\|depression. Under drought stress conditions(（5±1)%), the net photosynthetic rate reduced greatly with unobvious diurnal variation. The diurnal variations of transpiration rate and stomatal conductance presented double peak curves. The diurnal variation of intercellular CO2 concentration was opposite with that of net photosynthetic rate. Under soil water stress conditions, maximum photosynthetic rate, light saturation point, apparent quantum yield and water use efficiency of Leymus chinensis decreased, and light compensation point increased. Therefore, drought stress could reduce the photosynthetic ability of grassland vegetation, and result in a large decline of Hulunber meadow ecosystem productivity.