How to improve alpine grassland management level and achieve its sustainable use through reasonable utilization has been a hot research topic in the field of grassland ecology. In order to clarify the succession patterns of keystone species of alpine grassland plant communities under different herbivore assemblage grazing and their driving factors, this study set up grazing plots (Yak grazing alone, Tibetan sheep grazing alone, Yak and Tibetan sheep 1:2 mixed grazing, Yak and Tibetan sheep 1:4 mixed grazing, Yak and Tibetan sheep 1:6 mixed grazing) and enclosure plots with different herbivore assemblages based on moderate grazing intensity, then systematically analyzed the changes of plant community characteristics and soil physicochemical properties under different herbivore assemblage after 8 years of continuous grazing treatment, hope to determine the optimal grazing combination. The results showed that:(1) Grazing significantly reduced plant coverage, but the effect of different herbivore assemblages on plant coverage and species richness was not significant. (2) The plant community characteristics during 1:2 mixed grazing were similar to those of plant communities with enclosure. At the same time, the plant community during 1:2 mixed grazing pocessed the highest community stability and organization level. (3) The keystone species showed obvious succession under different herbivore assemblages. The keystone species was Kobresia humilis under the treatments of yak only and yak:sheep as 1:2. The keystone species was Leymus secalinus under the treatments of enclosure and yak:sheep as 1:6. It was Stipa sareptana and Potentilla acaulis in sheep only and yak:sheep as 1:4, respectively. (4) Enclosure could significantly reduce soil bulk density, while Tibetan sheep grazing alone could significantly increase soil bulk density and soil available nitrogen and phosphorus content. (5) Soil available nitrogen, bulk density and water content were the key physical-chemical factors driving the changes of keystone species in plant communities under different herbivore assemblages. In summary, 1:2 mixed grazing of yak and Tibetan sheep under moderate grazing intensity is a best way to utilize alpine grasslands on the Tibetan Plateau. In addition, establishing effective associations between plant community species is another key way to improve the management level of alpine grasslands on the Tibetan Plateau compared to simply increasing the species richness or diversity of grasslands.