Clarifying the nutrient cycling processes in forest ecosystems of different densities is of great significance for forestry construction, as it provides insights into ecosystem nutrient cycling and aids in managing artificial forests. The study focused on Robinia pseudoacacia plantation forests in the Loess Plateau in China, selecting six stand densities for analysis(i.e., 1000, 1500, 1800, 2400, 3000, 3500 plants/hm²). The study analyzed the characteristics of soil and microbial C, N, P content, extracellular enzyme activity, and ecological stoichiometric ratios (soil, microbial biomass, extracellular enzyme). The results showed that: with the increase of stand density, (1) soil C, N contents and C ∶ P&N ∶ P ratios increased initially then decreased, being significantly higher at medium stand densities compared to others; the C ∶ N ratio showed an opposite trend, and P content did not significantly differ. (2) The soil microbial C, N, and P content showed fluctuating changes, C ∶ N ratio showed a significant decreasing trend, C ∶ P and N ∶ P ratios increased first and then decreased. (3) Four kinds of soil extracellular enzyme activities and their C ∶ N ratio increased first and then decreased, C ∶ P and N ∶ P ratios showed a trend of increasing. (4) The element ratio threshold (TER) showed that except for the 1800 plants/hm² density, the soil microbial in other densities was restricted by N. The soil extracellular enzyme activity and microbial biomass can self-regulate and exhibit good internal stability in response to changes in soil nutrients. Medium-density Robinia pseudoacacia forests, particularly those with 1800 trees/hm², were more conducive to nutrient cycling and favorable for the growth and reproduction of microorganisms. This study could provide scientific support for the rational management of artificial forests in the study area.