Abstract：Temperature and precipitation are key climatic factors regulating ecohydrological processes in arid and semi-arid regions of China. However, the impacts of their synergistic effects on soil microbial processes remain to be further clarified. Based on long-term observational data from the Naiman National Field Scientific Observation and Research Station, this study focused on meadow aeolian sandy soil in the Horqin Sandy Land to quantitatively evaluate the synergistic effects of temperature and precipitation on soil microbial biomass carbon. The results indicated that the synergistic effect of temperature and precipitation is mediated by soil moisture as a critical intermediate, which regulates the dynamics of soil microbial biomass carbon through the core pathway: “temperature-precipitation synergy—soil moisture response—soil microbial feedback”. Furthermore, a soil moisture content of 0.019 m3/m3 can serve as the critical threshold for distinguishing different types of temperature-precipitation synergistic interactions. Under the scenario of “increased temperature and increased precipitation”, rising temperatures dominate and enhance soil evaporation. Precipitation supplementation fails to offset evaporative losses, resulting in drought stress that inhibits the accumulation of microbial biomass carbon. In contrast, under the scenario of “decreased temperature and decreased precipitation”, reduced temperatures weaken evaporation, while decreased precipitation suppresses deep water infiltration—thus maintaining surface soil moisture at an optimal state and facilitating the accumulation of microbial biomass carbon. This study not only deepens the understanding of how the synergistic effects of multiple climatic factors modulate soil microbial processes in arid and semi-arid regions but also provides significant insights for elucidating soil carbon cycle processes in sandy land ecosystems.