Artemisia frigida communities on Chinese Inner Mongolia steppe have been grazed by sheep in enclosures for thirteen years under four grazing intensities: zero-grazing (0.00 sheep/hm²), light-grazing (1.33 sheep/hm²), medium-grazing (4.00 sheep/hm²), and heavy grazing (6.67 sheep/hm²). The plant biodiversity was investigated from July to August in 2002. The results showed that: (1) Under zero grazing, the vegetation density was significantly less than that under light, medium or heavy grazing. The difference of the vegetation density was not significant among the latter three grazing treatments. (2) After thirteen years, the dominant plant became Aneurolepidium chinense under zero grazing, but still was A. frigida under light and medium grazing. However, the population density of Carex duriuscula was largest under above three grazing treatments. Under heavy grazing, Potentilla acaulis became dominant and its population density was largest in the community. With grazing intensities increasing, the population density of indicators of the grassland degradation in different degraded phases first increased, and then decreased for A. frigida, while first sharply increased, then gradually increased, finally sharply increased again for P. acaulis. (3) Diversity and evenness indices were largest under medium grazing, but smallest under zero grazing, which showed that the plant diversity was highest under medium grazing, but lowest under zero grazing. Dominant indices were contrary to the above two indices. Community structure and plant diversity were controlled by the integrated influence of selective browsing of sheep, adaptive strategies of different plants to grazing, interspecific competition, co-evolution between animal and plant and physical and chemical characteristics of the soil changed by grazing. These findings were contributed to provide an insight into the successive theory of the degraded grassland under the continuous disturbance of grazing and provide a theoretical foundation for the protection and restoration of the degraded grassland.