The current body of research on using composite microbial inoculants in soil to promote straw decomposition, potassium solubility, phosphorus solubility, and crop growth is relatively limited. In this study, we developed a composite microbial inoculant, designated HC-2, through the enrichment of the field with two strains of Bacillus. The agent displays multifunctionality and promotes rot prevention, solubilization of potassium and phosphorus, growth stimulation, and disease suppression. To further investigate its effects on soil fertility and microbial communities, field experiments were conducted. The results led to the identification of the optimal preparation conditions for the HC-2 compound. In accordance with the aforementioned conditions, the agent demonstrated considerable cellulose-degrading enzyme activity, resulting in a 31.5% increase in the rate of straw degradation within 90 days. Furthermore, it exhibited pronounced effects on maize seedling growth and demonstrated exceptional potassium and phosphorus-solubilizing capabilities, and strong disease suppression ability. In conditions of straw return, HC-2 was observed to effectively improve soil fertility and optimize the soil micro-ecological environment, with the number of operational taxonomic units (OTUs) reaching 536. The abundance of Proteobacteria increased by 10.8%, while that of Acidobacteriota decreased by 10.93% and that of Bacillus increased by 1.05%. Additionally, soil transcriptional functions, signaling mechanisms, and inorganic ion transport metabolic functions were enhanced. This study demonstrates that the HC-2 microbial inoculant overcomes the limitations of single-strain agents and has significant application value in improving farmland soil quality.