The Qilian Mountains National Park is a crucial ecological barrier and pastoral region in China, rich in biodiversity but ecologically fragile and highly sensitive to climate change and human activities. This study examines grassland plant communities across the plateau’s latitudinal and longitudinal zones, focusing on species, functional diversity, and aboveground biomass. We calculated multi-year averages, trends, and variation coefficients for climate factors (temperature and precipitation) at each site, along with the landscape diversity index. Key findings include: (1) A warming and drying trend in southern Qilian Mountains National Park, with significant precipitation variation at wetland meadows; (2) Plant height and leaf cover respond similarly to climate factors and landscape diversity, with plant height being more sensitive; (3) Higher climate variability correlates with lower functional diversity and smaller plant size, suggesting that environmental instability reduces diversity; (4) Landscape diversity is positively correlated with species diversity and biomass, enhancing productivity; (5) Most plant diversity indices show a positive relationship with biomass, supporting the role of diversity in productivity. This study provides valuable insights into the factors driving changes in grassland plant diversity in the Qilian Mountains National Park, offering evidence to inform biodiversity conservation strategies in the face of climate change.