Moso bamboo (Phyllostachys edulis) is crucial for carbon sequestration in subtropical regions. Its distinctive rhizomatic growth allows it to continually extend into surrounding ecosystems, a process associated with the decline in biodiversity and the deterioration of ecosystem functions. Soil fungi, essential to global biogeochemical cycles, facilitate key processes like carbon fixation and decomposition, bridging soil carbon inputs and outputs. However, the influence of moso bamboo expansion on soil fungal community composition and function remains unclear. This study investigates the impact of moso bamboo expansion on soil fungal communities across different forest types within the expansion zone, including pure p. edulis forests (PE), mixed p. edulis-broadleaf forests (MEP, with 20%—30% broadleaf species), mixed p. edulis-broadleaf forests (MEB, with 60%—70% broadleaf species), and broadleaf forests (BL). Illumina MiSeq high-throughput sequencing and the FUNGuild functional annotation platform were utilized to assess the structure and functional guilds of soil fungal communities in these subtropical forests. The findings demonstrated that the OTU counts, Chao1 indices, and Shannon-Wiener indices of soil fungi in moso bamboo forests were markedly higher compared to those in mixed bamboo-broadleaf and evergreen broadleaf forests. PCoA analysis, coupled with Adonis and ANOSIM tests, revealed significant differences in soil fungal community structures among forest types (P = 0.001 and P = 0.002). A total of 13 fungal phyla, 50 classes, 125 orders, 283 families, and 619 genera were identified across all samples. Basidiomycota, Ascomycota, unclassified fungi (unclassified_k__Fungi), Mortierellomycota, Rozellomycota, and Mucoromycota comprised 99.17% of the relative abundance of soil fungi. With moso bamboo expanded, the relative abundances of Basidiomycota and Rozellomycota significantly decreased by 28.4% and 15.8%, respectively compared to evergreen broadleaf forests, while the relative abundance of Ascomycota and Mortierellomycota increased significantly. The predominant fungal functional group was saprotrophic, followed by symbiotic. Moso bamboo expansion led to a 19.21% increase in the proportion of saprotrophic fungi and a 31.72% decrease in symbiotic fungi. Within the saprotrophic group, wood-decaying fungi exhibited the highest relative abundance in the PE forest, whereas soil saprotrophic fungi dominated in the BL forest. In the BL forest, the relative abundance of ectomycorrhizal fungi increased significantly (P < 0.05). Mantel test analysis identified soil pH, organic carbon content, dissolved organic carbon, microbial biomass carbon, and microbial biomass nitrogen as key factors influencing fungal community diversity. Redundancy analysis further indicated that dissolved organic carbon content was the primary factor responsible for significant differences in soil fungal community structure (P < 0.05). In conclusion, the expansion of moso bamboo not only alters the composition of dominant fungal phyla and genera but also significantly impacts the relative abundance of saprotrophic and symbiotic fungal communities. These findings offer critical insights into the carbon dynamics of subtropical forests and the ecological management of moso bamboo.