This study was performed to gain an understanding of the time cumulative effects of grazing intensity on the diversity of species, life forms, functional groups, and biomass and their relationships on an alpine meadow. From 2012 to 2015, we conducted a controlled grazing trial with six grazing intensities on an alpine meadow located in the northeastern Qinghai-Tibet Plateau. We investigated the diversity and life form of species, functional group diversity (richness, evenness, and dominance were used as diversity indices), and aboveground biomass in four years. We used repeated measures and linear regression methods to analyze the effect of grazing intensities and grazing years on plant species diversity, life forms, and functional groups, as well as on the relationship between the diversities and biomass. The following results were obtained. (1) Grazing intensity and grazing year had significant effects on biomass, with the biomass significantly decreasing with an increase in grazing intensity. There was irregularity in biomass changes in different grazing intensity plots with progression of the grazing year. (2) There were significant effects of grazing year on abundance, evenness, and dominance in term of the plant species, life forms, and functional groups. However, the grazing intensity and grazing year had an interactive influence on the richness of plant species, and grazing intensity had a time cumulative effect on the richness of plant species. (3) In the plant diversities with different organizational levels under grazing disturbance, only the richness of plant species was significantly correlated with all diversity indices. The richness of plant species can be used as a representative index for measuring plant species diversity. (4) In the study of the relationship between diversity and biomass, the richness of plant species in the lighter grazing intensity plot (Plot 2) was significantly decreased with increasing biomass, whereas the richness of plant species, life forms, and functional groups showed no relationship with biomass in other grazing intensity plots. The evenness of life forms in the high intensity grazing plots (Plots 4, 5, and 6) significantly decreased with an increase in biomass. The dominance of life forms in the high intensity grazing plots (Plots 4, 5, and 6) significantly increased with an increase in biomass. In conclusion, the diversity of life forms can be used as an effective index for rapidly predicting plant biomass under grazing disturbance because this diversity is the most sensitive to biomass variation.