Soil microbes are the primary drivers of soil organic matter and nutrient cycling. It is critical to study carbon and nitrogen variations in soil microbial biomass, as well as their steady-state properties and inherent long-term response mechanisms to environmental variables. Soil microbial biomass carbon and nitrogen (MBC and MBN), organic carbon, total nitrogen, total phosphorus, readily oxidizable carbon (ROC), available nitrogen, pH, temperature, and water content have been continuously monitored in a subtropical evergreen broad-leaved forest soil for 10 years in this study. The ANOVA, correlation and regression analysis, and steady-state analysis were employed to investigate the interannual variations and steady-state characteristics of MBC and MBN and the main factors influencing them. The results demonstrated that (1) the MBC and MBN contents varied between 171.32 to 358.45 mg/kg and 25.90 to 54.08 mg/kg in the dry season and from 394.01 to 507.97 mg/kg and 68.40 to 88.05 mg/kg in the rainy season, respectively. Significant inter-annual variation of MBC content in both dry and rainy seasons was detected (P < 0.05). Whereas, the content of MBN only changed significantly between years in the dry season (P < 0.05). The MBC and MBN contents in the rainy season were significantly higher than those in the dry season (P < 0.01) and it's twice as high in the dry season than in the rainy season. (2) There was a significantly positive correlation between MBC and MBN in both dry and rainy seasons (P < 0.05). Both MBC and MBN were positively correlated with ROC and AN in the dry season. Meanwhile, MBN was significantly positively correlated with TP (P < 0.05) and SOC (P < 0.01). In the rainy season, only MBN was significantly positively correlated with SOC (P < 0.05). (3) In the dry season, the variety of the MBC content was mainly influenced by ROC (P < 0.05) and AN (P < 0.001), whereas MBN variation was primarily influenced by AN (P < 0.05). AN (P < 0.05) dominated the variation of MBC in the rainy season, while TP (P < 0.05) and SOC (P < 0.05) were the dominant factors of MBN variation. Taken together, these results suggested that AN (P < 0.001) and SOC (P < 0.001) were the soil factors with the highest contribution to the variation of MBC and MBN contents. (4) The homeostasis index of MBC and MBC/MBN were absolutely steady-state on an annual scale (P > 0.05), and MBN (P=0.685) was an absolutely steady-state in the rainy season. In contrast, the MBN homeostasis index of soil was found to be in an unstable state (P < 0.01, H > 1) in the dry season. Soil microbial quotient was significantly higher in the rainy season than that in the dry season (P < 0.01), implying the quality of soil organic matter and efficiency of nutrient utilization were higher in the rainy season. In summary, the MBC and MBN were significantly influenced by seasonal turnover, and the dominant soil factors on MBC and MBN variation were SOC and AN. Soil MBN was in an unstable state due to water limitations in the dry season.