Soil microbial carbon use efficiency (CUE), also known as microbial growth efficiency, is the efficiency of microorganisms to convert carbon absorbed into biomass. It is a key ecophysiological parameter for regulating soil carbon sequestration and turnover, carbon mineralization, and greenhouse gas emissions. Knowledge on the variations and controlling mechanisms of soil microbial CUE is important to understand the soil C cycle and its response to global changes. In this study, we clarified the concept and measurements of the soil microbial CUE, and summarized the advances in the fields of its variations and influencing factors. Based on those results, we found that soil microbial CUE was generally expressed as the ratio of microbial growth to uptake, and measured by the change of microbial growth rate, biomass, substrate uptake rate or substrate concentration. The soil microbial CUE varied greatly in the range of 0.2-0.8 rather than a constant. The variations were attributed to thermodynamic constraint, environmental condition, nutrient quality and availability, stoichiometric control and microbial community composition. Future research on soil microbial CUE should focus on the quantification of neglected metabolic components, exploration of the interactive effects of the biological and abiotic factors, and integration of the dynamic physiological responses of microorganisms into soil C cycle models.