This study investigates the enhancement of methane production in anaerobic co-digestion (AcoD) through the introduction of exogenous CO₂ and the application of bamboo biochar. Exogenous CO₂ boosts biogas yield by providing an additional carbon source, which requires optimized solubility and pH buffering to ensure effective methanation. Biochar serves as an electron shuttle and pH stabilizer, facilitating CO2 solubility and syntrophic interactions that enhance microbial stability. When combined, biochar and CO₂ (R2) achieved a significant synergistic effect, increasing specific methane production (SMP) by 42.56% compared to the control (R0). Independent additions of biochar (R1) and CO₂ (R3) also improved SMP, with increases of 35.50% and 28.01%, respectively. This enhancement is likely due to the elevated activity of homoacetogenic bacteria and hydrogenotrophic methanogens, with increased acsB gene expression 2.4-fold with biochar + CO₂ and 1.5-fold with CO₂ alone compared to the control. Additionally, biochar facilitated syntrophic metabolism mediated by Cytochrome-C, promoting electron transfer. The study also demonstrated that biochar and CO2 could enhance enzyme activity, including acetyl-CoA synthase, mhpF, and mhpE. Such improvements bolster AcoD efficiency and promote resource recycling within the circular economy framework.