Grassland plant diversity is a core issue in grazing ecology. We examined the effects of grazing intensity on plant diversity across four levels of organization (plant individual, species, functional group, and community) in Stipa breviflora desert grassland using a six-year sheep grazing experiment in Sonid Right Banner, Xilingol, Inner Mongolia Autonomous Region of China. Species richness was greater in the no-grazing plot than in the moderate- and heavy-grazing plots. The differences were due to two functional groups, perennial forbs and annual forbs. The relative weight of each functional group was not affected by grazing intensity. At the community scale, grazing intensity did not significantly affect richness. However, the Simpson ecological dominance index, Shannon-Wiener diversity index, and Pielou evenness index of no-grazing plot plant communities were greater than those of the moderate-grazing plot and significantly greater than those of the heavy-grazing plot (P < 0.05). At the functional group scale, the diversity indices of perennial grasses and annual herbs had no significant responses to grazing. The diversity index of no-grazing plot plants was greater than those of moderate- and heavy-grazing plots; the moderate-grazing plot diversity index was higher than that of the heavy-grazing plot. Stipa breviflora desert grassland communities were all unstable to some degree, but the moderate-grazing plot stability was greater than those of the no- and heavy-grazing plots. Our research suggests that an increase in grazing intensity will decrease plant diversity at multiple levels of organization in Stipa breviflora desert grasslands. However, moderate-grazing grasslands were relatively stable. Therefore, the balance of plant diversity and stability will be an important way for developing the regional scientific grazing intensity.