[Objective] To optimize the anaerobic digestion process for methane production of hulless barley straw with mixed addition of biochar and Fe₃O₄ and investigate the feasibility of mixed addition of biochar and Fe₃O₄ in the anaerobic digestion of lignocellulosic waste. [Methods] We employed single factor and response surface experiments to optimize the anaerobic digestion process for methane production of hulless barley straw with mixed addition of biochar and Fe₃O₄. Metagenomics was employed to analyze the microbial community structure and methane production pathway during digestion. [Results] The optimal digestion conditions were 6.32% total solids, biochar-to-Fe₃O₄ ratio of 6.83:3.17, and inoculation ratio (volatile solids ratio of inoculum to hulless barley straw) of 2.51. Under these conditions, the measured value of cumulative methane production based on volatile solids was 269.04 mL/g, with a relative error of less than 5% from the predicted value (265.95 mL/g), which indicated that the model was effective. The mixed addition treatment under the optimized conditions increased the methane production of hulless barley straw (P<0.05), with the effect comparable to that of the chemical pre-treatment. Meanwhile, the treatment increased the acetic acid content while reducing the accumulation of propionic and butyric acids. Metagenomic analysis showed increases in the relative abundance of bacterial taxa such as unclassified_Bacteroidota, unclassified_Bacteria, Clostridium, and Fibrobacter, as well as acetotrophic methanogens such as Methanosarcina and Methanothrix, which promoted the utilization of acetic acid and enhanced the direct interspecies electron transfer (DIET) between microorganisms. The mixed addition of biochar and Fe₃O₄ in the anaerobic digestion system enhanced the acetotrophic methanogenic pathway, thereby enhancing methane production. [Conclusion] The response surface methodology can optimize the anaerobic digestion process for methane production of hulless barley straw with mixed addition of biochar and Fe₃O₄. The mixed addition of biochar and Fe₃O₄ enables efficient production of biomethane and environmentally friendly treatment of lignocellulosic waste.