This study aims to explore the response mechanisms of alpine grassland ecosystems to climate change in terms of vertical zonation, specifically focusing on the temperate steppe on the southern slope of the Qilian Mountains. We conducted a comprehensive analysis of the spatio-temporal evolution characteristics of this temperate steppe over the past 35 years, quantitatively evaluating the relative importance of key climatic factors affecting changes in the area of the temperate steppe, and revealing the nonlinear response relationships and thresholds between area changes and climatic fluctuations. The findings indicate that: (1) Over the past 35 years, the area of the temperate steppe on the southern slope of the Qilian Mountains has shown an increasing trend, at a rate of 28.3 km²/a. The areas of expansion are mainly concentrated around Qinghai Lake, in river valleys, and in mountainous regions above 3000 m elevation on the southern Qilian Mountains, while the areas of reduction are chiefly located in sandy regions east of Qinghai Lake and in areas below 3000 m elevation. Vertically, the temperate steppe has migrated to elevations above 2900 m, as well as to slopes exceeding 25°, flat lands, and semi-sunny slopes. (2) Temperature, precipitation, and wind velocity are the main climatic factors affecting the area changes of the temperate steppe, with the contribution rates of the climatic factors ranked as September precipitation>wind velocity>March temperature>April temperature>August temperature>July temperature. Of these, September precipitation has the greatest contribution rate (20.6%), succeeded by wind velocity (18.5%). (3) Climatic factors, including precipitation, wind velocity, and temperature, collectively impact the area change of temperate grasslands in a complex, nonlinear fashion. When the precipitation change rate in this area is within 0.2-0.5 mm/a in September, the wind speed change rate is within -0.005--0.0025 m s^-1 a^-1, the temperature change rate in March is within 0.01-0.02 ℃/a, the temperature change rate in April and August is within 0.02-0.06 ℃/a, and the temperature change rate in July is within 0.02-0.03 ℃/a, the increasing trend of the area of temperate steppe is the most significant. Under other circumstances, the increasing trend of the area slows down, but no negative growth is presented. These discoveries will offer scientific decision-making support for the protection, restoration, and management of temperate steppe vegetation in the southern Qilian Mountain region.