In order to clarify the water use strategy and adaptive mechanism of plants, this study took Stipa breviflora desert steppe of Siziwang Banner, Inner Mongolia as the research object, and set four grazing gradients: control (CK), light grazing (LG), moderate grazing (MG) and heavy grazing (HG). The stocking rates were 0, 0.93, 1.82 and 2.71 sheep units per hectare per year, respectively. The height, coverage, density, aboveground biomass and soil physicochemical properties of the established species S. breviflora were investigated, and the water use efficiency of S. breviflora was measured by stable carbon isotope method and infrared photophotoapparatus method. The objective was to elucidate the response of water use efficiency of S. breviflora under different grazing intensities and its influencing factors. The results showed that: (1) Grazing had significant effects on coverage, density and aboveground biomass of S. breviflora. With the increase of stocking rate, S. breviflora spread and its distribution area increased, especially under moderate grazing condition. (2) With the increase of grazing intensity, soil moisture content increased significantly compared with the control, soil total nitrogen content increased first and then decreased, soil available potassium showed a trend of decreasing, but had no significant effect on soil total carbon content and pH, indicating that moderate grazing could increase soil moisture content and promote the accumulation of soil nitrogen content, but grazing would lead to the decrease of soil available potassium. (3) With the increase of grazing intensity, long-term water use efficiency (WUE_l) showed a V-shaped trend, while instantaneous water use efficiency (WUE_t) and intrinsic water use efficiency (WUE_i) showed a decreasing trend. (4) Correlation analysis showed that grazing intensity was significantly positively correlated with S. breviflora density and aboveground biomass, soil total nitrogen content was significantly positively correlated with organic carbon, pH and WUE_i, and WUE_t and WUE_i were significantly positively correlated. The intrinsic water use efficiency of S. breviflora was closely related to soil organic carbon content. The results showed that heavy grazing resulted in the fragmentation of S. breviflora and increased the population diffusion area, which was the direct reason for the improvement of long-term water use efficiency of S. breviflora. The decrease of instantaneous water use efficiency with the increase of grazing intensity may be caused by the decrease of intrinsic water use efficiency of S. breviflora.