The stoichiometry of soil microbial biomass is closely tied to the terrestrial cycling of carbon (C) and nutrients. Those stoichiometry influences not only soil fertility but also global change mitigation efforts. Ecologically fragile areas are commonly located in the ecotones between different ecosystems, with poor stability, weak ability to resist disturbance, and easy degradation. Yet the biogeographical patterns and potential mechanistic drivers of soil microbial biomass stoichiometry and its homeostasis in fragile ecosystems remain unclear. In this study we set three transects in typical ecologically fragile areas of China from which topsoil (0 - 20 cm depth) samples were collected to measure their microbial biomass carbon (MBC), nitrogen (MBN), and phosphorus (MBP). The results revealed much lower MBC (190.1 mg kg(-1)), MBN (20.5 mg kg(-1)), and MBP (7.1 mg kg(-1)) concentrations than their corresponding global and Chinese averages, but higher MBC:MBN (9.7), MBC:MBP (48.4), and MBN:MBP (4.3) ratios. The stoichiometric homeostasis (H, calculated by nonlinear fitting of microbial and soil stoichiometry) of MBC, MBN, and MBP concentrations ranged from 1.4 to 2.3, being weakly plastic to weakly homeostatic, while their ratios were generally homeostatic (with H ranged from 4.2 to 45.5). The MBC:MBN:MBP ratio was approximately 48:4:1. Overall, soil microbes were vulnerable to soil nitrogen and phosphorus limitations, compared to soil carbon. Soil MBC:MBN:MBP stoichiometry changed nonlinearly along the latitudinal gradient. Soil chemical and vegetation properties contributed the majority of variance explanations (> 62 %) to microbial biomass stoichiometry. Altogether, these findings provide new insight into the patterns and homeostasis of soil MBC:MBN:MBP stoichiometry in fragile ecosystems.