Soil microbes are directly involved in soil organic carbon (SOC) decomposition, yet the importance of microbial biodiversity in regulating the temperature sensitivity of SOC decomposition remains elusive, particularly in alpine regions where climate change is predicted to strongly affect SOC dynamics and ecosystem stability. Here we collected topsoil and subsoil samples along an elevational gradient on the southeastern Tibetan Plateau to explore the temperature sensitivity (Q10) of SOC decomposition in relation to changes in microbial communities. Specifically, we tested whether the decomposition of SOC would be more sensitive to warming when microbial diversity is low. The estimated Q10 value ranged from 1.28 to 1.68, and 1.80 to 2.10 in the topsoil and subsoil, respectively. The highest Q10 value was observed at the lowest altitude of forests in the topsoil, and at the highest altitude of alpine meadow in the subsoil. Variations in Q10 were closely related to changes in microbial properties. In the topsoil the ratio of gram-positive to gram-negative bacteria (G+:G−) was the predominant factor associated with the altitudinal variations in Q10. In the subsoil, SOC decomposition showed more resilience to warming when the diversity of soil bacteria (both whole community and major groups) and fungi was higher. Our results partly support the positive biodiversity-ecosystem stability hypothesis. Structural equation modeling further indicates that variations in Q10 in the subsoil were directly related to changes in microbial diversity and community composition, which were affected by soil pH. Collectively our results provide compelling evidence that microbial biodiversity plays an important role in stabilizing SOC decomposition in the subsoil of alpine montane ecosystems. Conservation of belowground biodiversity is therefore of great importance in maintaining the stability of ecosystem processes under climate change in high-elevation regions of the Tibetan Plateau.