［Objective］Artificial young forests are an important component of forest carbon sinks in Guangxi. Clarifying the dominant factors of their carbon sink function is the basis for formulating forest carbon sequestration management strategies. This study took the artificial young forests in the Liuzhou section of the Liujiang River Basin as the research object, investigated the effects of stand structure and soil site conditions on vegetation carbon density, and identified the key influencing factors.［Methods］Stand data were obtained through field plot surveys. Vegetation carbon density of five types of artificial young forests was estimated using biomass equations and carbon content coefficients. Key factors were identified using a random forest model, and Pearson correlation analysis was further applied to reveal the relationships between vegetation carbon density and each factor.［ Results］ (1) Vegetation carbon density differed significantly among forest types, with broad-leaved forests having the highest (84.58±65.72 t·hm-2) and shrublands the lowest (15.93±2.20 t·hm-2). (2) Stand structure and site conditions jointly dominated vegetation carbon density patterns. DBH was the most critical stand factor with the highest contribution rate (39.70%); soil layer thickness (25.40%), soil type, and humus layer thickness were important site factors. Vegetation carbon density increased significantly with increasing DBH and soil layer thickness (p<0.05). (3) High understory species diversity does not necessarily lead to high carbon storage.［Conclusion］Vegetation carbon density of artificial young forests in the Liujiang River Basin is primarily controlled by stand structure and site conditions, with DBH and soil layer thickness being the key factors constraining carbon accumulation. The influence of understory diversity is limited, as the biomass accumulation of a few dominant tree species dominates carbon density patterns, with the "selection effect" suppressing the "complementarity effect" of understory diversity. These results suggest that enhancing carbon sequestration in plantations of the Liuzhou section requires optimizing tree species composition, promoting DBH growth, and improving soil conditions through measures such as adjusting the proportion of broad-leaved species, timely tending and thinning, and retaining litter.