Biochar was added to a mesophilic anaerobic digester to promote syntrophic volatile fatty acids (VFAs) oxidation and methane production from complex organic wastes. Compared with conventional operation, biochar addition effectively shortened the lag time by 27.5-64.4% and increased the maximum methane production rate by 22.4%-40.3%. With a biochar dosage of 15 g/L, the system performed well under an organic loading rate as high as 3 g substrate/g inoculums. Biochar showed a remarkable buffering capacity to alleviate pH decrease caused by VFAs accumulation. In order to gain knowledge on associated mechanisms, a specific experiment was conducted using butyrate as substrate. It was identified that syntrophic degradation of butyrate to acetate occurred under high H-2 partial pressure. By microbial community analysis, it was further revealed that biochar addition brought about the enrichment of Anaerolineaceae and Methanosaeta, typical microorganisms for direct interspecies electron transfer.