Abstract: This study established three planting density treatments—T1 (1050 trees·hm⁻²), T2 (1200 trees·hm⁻²), and T3 (1350 trees·hm⁻²) in Eucalyptus plantations in the dry-hot valley area of Guangxi. It systematically analyzed the changes in soil chemical properties, key hydrolase activities, and microbial biomass along with their stoichiometric characteristics. The results showed that (1) Soil pH decreased initially and then increased with increasing planting density, reaching the lowest value under T2, while the contents of organic matter, total nitrogen (TN), total phosphorus (TP), alkali-hydrolyzable nitrogen (AN), and available phosphorus (AP) peaked in this treatment. Total potassium (TK) decreased continuously with increasing density, whereas available potassium (AK) showed an opposite trend. (2) The activities of soil β-glucosidase (βG), N-acetyl-β-D-glucosaminidase (NAG), leucine aminopeptidase (LAP), and acid phosphatase (ACP) were highest under T2. Microbial biomass carbon (MBC), nitrogen (MBN), and phosphorus (MBP) increased continuously with density and reached their maximum values under T3. The microbial C:N ratio was highest in T1, while the microbial C:P and N:P ratios were highest in T2. (3) Correlation analysis indicated that organic matter, TN, AN, AP, and AK were significantly positively correlated with enzyme activities and MBC and MBN contents, whereas TK was significantly negatively correlated with βG, LAP, ACP, MBC, MBN, and MBP. The C:N ratio decreased significantly with increases in organic matter, TN, TP, AN, AP, and AK, while the N:P ratio was significantly positively correlated with organic matter, TN, TP, and AP, suggesting that phosphorus may become a potential limiting factor for microbial growth under high-nitrogen conditions. Planting density synergistically influences enzyme activities and microbial functions by regulating soil nutrient availability, with the three components exhibiting a tightly interconnected "soil nutrient—enzyme activity—microbial biomass" association network Among the treatments, T2 (1200 trees·hm⁻²) was most favorable for maintaining soil fertility and enhancing enzyme activity, while T3 (1350 trees·hm⁻²) promoted greater microbial biomass accumulation.