Display omitted •Biochar packed reactor improved the CH4 yield and stability of tar degradation.•Biochar alleviated the inhibition of tar solution to anaerobic digestion.•Biochar prolonged the retention time of most organic matters in tar solution.•Packed biochar promoted the syntrophic growth of different microbes. Tar generated as a by-product during biomass gasification contains a high concentration of refractory organic matters. In this study, a hybrid upflow anaerobic sludge-biochar bed reactor was established for tar treatment, and the methane yield was 120–154 NmL-CH4/g-CODinf, 20–30% higher than the control reactor. COD removal and methane production significantly decreased in both reactors when the influent tar concentration was doubled from 4954 mg-COD/L to 9964 mg-COD/L. When the influent concentration was reduced, the biochar packed reactor showed a faster recovery. Batch tests confirmed that higher tar concentration inhibited methane production and induced longer lagphase. Biochar addition effectively relieved the inhibition and prolonged the retention of organic matters. SEM observation and 16S rRNA analysis suggested that biochar also acted as the microbe’s carrier, and promoted the growth of some microbes. The results of this study provide new ideas for tar treatment.