Soil microorganisms serve as pivotal biogeochemical engines in forest ecosystems. In response to ecological challenges, particularly soil degradation associated with intensive management of pure coniferous plantations, the transition toward mixed coniferous–broadleaved forests has emerged as a strategic approach to enhance the functional integrity and resilience of artificial forest ecosystems. However, the mechanisms by which such mixed forests influence soil microbial community assembly remain insufficiently synthesized. This review adopts a mechanistic framework centered on differential above- and belowground resource inputs, soil habitat filtering, microbial community responses, and downstream ecosystem functional outcomes. We critically synthesized recent advances regarding how mixed coniferous-broadleaved forests shape the composition, structure, and functioning of forest soil microbial communities. Specifically, we addressed: 1) non-additive effects arising from interspecific differences in litter quantity and quality, decomposition dynamics, and root-zone processes, including spatial complementarity of root systems, rhizodeposition-mediated biochemical interactions, and mycorrhizal network connectivity, which collectively generate heterogeneous resource input pathways; 2) how these differentiated inputs improve soil physical architecture (e.g., aggregation, porosity) and modulate chemical conditions (e.g., pH, nutrient availability, redox status), thereby imposing selective environmental filtering pressures on microbial taxa; 3) corresponding microbial responses, including shifts in total biomass, spatial redistribution of extracellular enzyme activities, restructuring of taxonomic and functional community composition, and increased complexity of interspecific interaction networks. Furthermore, we emphasized how mixed stand structural complexity and tree species diversity amplify these cascading effects. Finally, we identified critical knowledge gaps in current research, particularly concerning underlying biological mechanisms (e.g., allelopathic interactions, deterministic versus stochastic community assembly rules), temporal and spatial scaling of microbial responses, and integration across hierarchical ecological levels. We proposed that future progress hinges on synergistic integration of multi-omics approaches, stable isotope probing, long-term in situ monitoring, and process-based modeling—ultimately enabling the development of a predictive framework linking plant functional traits to microbial functional attributes. Such a framework would provide a robust scientific foundation for precision-oriented management of mixed coniferous–broadleaved forests aimed at enhancing carbon sequestration and sustaining soil fertility.