Abstract: To enhance the production of key precursors (primarily lactate and acetate) for medium-chain fatty acid (MCFA) synthesis in the anaerobic fermentation of agricultural waste, this study employed a precision regulation strategy involving bioaugmentation with functional microbial consortia. With the growing demand for high-value utilization of biomass resources, the conversion of lignocellulosic wastes such as crop straw into MCFA (e.g., caproate and caprylate) has emerged as a frontier direction in the field of renewable energy. However, this conversion process typically relies on the hydrolysis and acidification stage to provide sufficient and appropriately proportioned volatile fatty acid precursors for subsequent chain elongation. Notably, when lactate and acetate coexist at a specific molar ratio, they can significantly promote the efficient synthesis of caproate. Therefore, directionally regulating the products of hydrolysis and acidification to enhance the synergistic production of lactate and acetate is key to overcoming the bottleneck of the overall process. Based on this, a highly efficient lignocellulose-degrading microbial consortium (designated WS-1) and a specialized lactate-acetate-producing functional consortium (designated RS-3) were isolated and selected for this study. In a 90-day continuous fermentation experiment, four treatment groups were systematically established for comparison: addition of WS-1 alone (Group A), addition of RS-3 alone (Group B), combined addition of WS-1 and RS-3 (Group C), and a blank control without functional consortium inoculation (Group D). The experimental results showed that Groups B and C, which were inoculated with the RS-3 consortium, exhibited the most prominent treatment effects. In these two groups, the total concentration of lactate and acetate reached up to 24.45 g/L, and their molar ratio was stably maintained within the ideal range of 2:1 to 3:1, creating optimal substrate conditions for the subsequent chain elongation reaction. Further analysis of the microbial community structure revealed that this treatment significantly enriched key genera related to substrate degradation and acid production, such as Romboutia, Lactobacillus, Clostridium, and Turicibacter, and simultaneously significantly enhanced the activities of key enzymes in the relevant metabolic pathways. When the acidified liquids produced from the above groups were used in the subsequent chain elongation stage, the optimal Groups B and C achieved a caproate yield as high as 10.68 g/L, representing an increase of approximately 103% compared to the control group (Group D). This study confirms that the directional introduction of a functionally defined lactate-acetate-producing microbial consortium can effectively regulate the targeted production of hydrolysis and acidification products from complex agricultural waste, achieving dual optimization of precursor yield and composition. This bioaugmentation strategy provides a key and feasible substrate optimization scheme for the efficient synthesis of MCFA through multi-stage fermentation using diverse wastes as raw materials, demonstrating significant application potential.