Abstract: Grassland ecosystems play pivotal roles in carbon cycling, water regulation, and biodiversity conservation, yet they are highly sensitive to environmental change. Focusing on the grasslands of the Hulun Lake Basin, we combined field surveys with meteorological data to examine community structure, temporal dynamics of species diversity, and their responses to environmental drivers in meadow steppe and typical steppe. The results show that: (1) species diversity is higher in meadow steppe than in typical steppe, but both exhibited degradation from 2022 to 2024: aboveground biomass decreased by 42% and 79%, respectively, and vegetation cover showed annual declines reaching 16% and 22%; (2) in typical steppe, the Shannon–Wiener diversity, Simpson diversity, and Pielou’s evenness indices increased slightly during the monitoring period but remained lower than in meadow steppe, whereas the Patrick richness index declined year by year; and (3) environmental response analyses and structural equation modeling (SEM) indicate that soil conditions are the primary drivers of grassland diversity across the study region, while meteorological factors predominate under extreme conditions. At the vegetation-type scale, meadow steppe is more strongly influenced by meteorological factors, whereas typical steppe depends more on soil conditions. These findings reveal type-specific response mechanisms of plant species diversity to environmental factors and provide a scientific basis for the conservation and management of grassland ecosystems in the Hulun Lake Basin.