The allometric relationships between growth traits are critical to trees’ fitness, yet the mechanisms by which slope position affects tree growth and allometry remain poorly understood. This study examined growth traits and their allometric relationships in an 8-year-old Calocedrus macrolepis plantation in southwest China across three slope positions (upslope, mesoslope and downslope). The measured growth traits included tree height (H), diameter at breast height (D), crown size (Crown), wood volume (V) and height under branch (HUB). The study also explored spatial variations in soil properties and microbial communities. Results showed that slope position altered allometric growth pattern, with larger allometric exponents at downslope for H, D and V relative to Crown and HUB, suggesting improved wood growth. Soil nutrient levels (nitrogen, phosphorus and available potassium) and microbial diversity, particularly the relative abundance of bacterial phyla such as Actinobacteria and Chloroflexi, were greater at mesoslope and downslope. Our study identified phosphorus and potassium as key drivers of enhanced allometric relationships. Functional groups such as Endomycorrhizal and Ectomycorrhizal fungi, and functional groups involved in nitrogen cycling (Nitrogen respiration, Nitrate respiration), were strongly correlated with allometric exponents for D, V and Crown relative to HUB, suggesting their role in supporting structural growth and canopy expansion. These findings emphasize that variations in soil nutrients and microbial communities across slope positions regulate tree growth and allometry, with bacterial communities exerting a stronger influence than fungi. These insights contribute to sustainable forest management, particularly in optimizing planting site selection for improved tree growth in mountainous regions.