Abstract: High lignocellulose content in tomato stalks hinders their resource utilization. To achieve rapid humification of tomato stalk lignocellulose, we established two treatments with tomato stalks as the primary materials: fine-crushed tomato stalks (FC) and coarse-crushed tomato stalks (CC). We then monitored the changes in physicochemical properties, humic substance content, and bacterial community structure during composting to investigate the effects of different particle sizes on lignocellulose degradation, humic substance formation, and bacterial communities. Results showed that the FC treatment significantly increased the maximum composting temperature (64.53 ℃) and prolonged the thermophilic phase (6 days). At maturity, the total nitrogen (TN) content in the FC treatment (2.48%) was significantly higher than that in the CC treatment (2.08%). Compared with the CC treatment, the FC treatment significantly reduced the cellulose and hemicellulose content by 8.81% and 21.17%, increased the humic substance (HS) content by 22.15%, enhanced the humification degree, and reduced the NH₃ and N₂O emissions by 28.18% and 24.86%, respectively. During the heating and thermophilic phases, the FC treatment showed higher relative abundance of lignocellulose-degrading and humification-promoting bacterial groups, including the phyla Actinobacteriota and Firmicutes, as well as the genera Paucibacter, Cerasibacillus, and Pseudogracilibacillus. Co-occurrence network analysis revealed higher modularity and tighter connections in the bacterial community in the FC treatment, indicating that fine crushing promoted substrate homogenization and drove the formation of specialized cooperative modules among functional bacteria. Mantel test analysis further demonstrated significant correlations between cellulose, hemicellulose, lignin contents and α-diversity, β-diversity, as well as core bacterial communities in the FC treatment, confirming that this modular division synchronously optimizes lignocellulose degradation efficiency and humic substance formation. In summary, fine crushing of tomato stalks accelerates lignocellulose degradation, enhances humic substance production, and optimizes microbial community structure, thereby effectively improving the quality and efficiency of tomato stalk composting and providing a scientific basis for efficient resource utilization of tomato stalks.