Abstract: Microorganisms are involved in the transformation and circulation of soil nutrients, and are also sources and sinks of soil nutrients. These processes and the abundance of soil microorganisms are sensitive to environmental changes. Therefore, changes in the abundance of microorganisms are an important index of changes in soil quality. In this study, grassland soil samples collected at altitudes of 1191-2656 m in Chabuchaer County, Ili, Xinjiang, were analyzed by chloroform fumigation extraction to determine differences in the quality and activity of soil microorganisms among different altitudes. The results showed that biomass carbon (21.05-331.58 mg/kg) and biomass nitrogen (8.75-95.61 mg/kg) were higher at the soil surface than in middle and lower layers, and differed significantly with altitude and among soil depths (P<0.05). The microbial biomass increased with increasing altitude from 1191 m to 2216 m (331.58 mg/kg, 95.61 mg/kg) and then significantly decreased as the altitude increased from 2216 m to 2656 m (21.05 mg/kg, 12.41 mg/kg). This may have resulted from changes in environmental and climatic conditions such as precipitation and the types and quantities of vegetation. Correlation analyses showed that soil biomass carbon was positively correlated with soil nitrogen and the carbon∶nitrogen ratio (P<0.05). Soil microbial biomass was positively correlated with moisture content at 0-20 cm depth (P<0.05, r=0.755^*) and with altitude, organic matter, bulk density, and available nitrogen (P<0.05). Soil microbial biomass showed negative correlations (P<0.05) with soil pH, total nitrogen, and the carbon∶nitrogen ratio. At the soil depth of 20-40 cm, soil microbial biomass was significantly positively correlated with available nitrogen (P<0.05, r=0.829^*). Soil microbial biomass showed positive correlations with altitude and total nitrogen (P<0.05), and a negative correlation (P<0.05) with the carbon∶nitrogen ratio. At the soil depth of 40-60 cm, soil microbial biomass showed negative correlations with altitude and the carbon∶nitrogen ratio (P<0.05), and positive correlations (P<0.05) with organic carbon, available nitrogen, total nitrogen. These results showed that soil microbial biomass responds to changes in soil fertility; therefore, it can be used as a biological index of soil fertility.