Abstract: To elucidate the patterns and drivers of age-related changes in soil microbial biomass carbon (MBC), nitrogen (MBN), and phosphorus (MBP) content and their ecological stoichiometry in Haloxylon ammodendron plantations, was conducted in the Minqin County ecological public welfare forest area, Gansu Province, China. Soil samples were collected from 0-60 cm (0–20, 20–40, and 40–60 cm) depths under Haloxylon ammodendron plantations of five different ages (3, 6, 10, 20, and 31 ages). The contents of MBC, MBN, and MBP were determined, and their stoichiometric ratios (MBC:MBN, MBC:MBP, MBN:MBP) were calculated. The results showed that the average contents of MBC, MBN, and MBP were 76.61, 28.17, and 11.01 mg/kg, respectively, with corresponding average stoichiometric ratios of 2.70, 6.23, and 2.39. The contents of MBC, MBN, MBP and their stoichiometric ratios increased significantly with plantation age (P < 0.05). Stand age and soil depth had significant individual effects on the contents of MBC, MBN, MBP and the ratios of MBC:MBN and MBN:MBP (P < 0.05). Their interaction significantly affected MBC and MBN contents but had no significant effect on the stoichiometric ratios. MBC, MBN, and MBP contents were significantly positively correlated with soil organic carbon and total nitrogen (P < 0.05), but negatively correlated with total phosphorus (P < 0.05). Redundancy analysis indicated that soil organic carbon was the most important factor, explaining 19.63% and 13.89% of the variations in microbial biomass content and stoichiometry, respectively. The study provides data support for the interaction and mechanism of soil microbial nutrient content in saxaul forests in desert oasis regions.