Coal gasification wastewater (CGW) is one of the most prevalent industrial effluents in China. It contains hundreds to thousands of milligram phenolic compounds per liter, becoming an important threat to the ecological environment. This study was conducted to evaluate the effectiveness of bioaugmentation with phenol-degrading bacteria (PDB) on improving start-up and stability of sequencing biofilm batch reactor (SBBR) for CGW treatment. The augmented SBBR, with a shortened start-up time of 13 days, showed considerably higher pollutant removal. The superiority of augmented SBBR persisted during the whole operation, with COD, total phenols (TPh) and NH3-N efficiencies of 86%, 89% and 35%, respectively, compared to 70%, 75% and 22% for non-augmented SBBR. Under the inhibition of a high concentration of phenolic compounds, the augmented SBBR presented a stronger resistant capability and less recovery time due to inoculating additional PDB. Microbial community analysis revealed that 13.9% of PDB in activated sludge and 10.8% in biofilm as dominant populations, which in turn determined the overall performance of the biosystem, resulting in an efficiently augmented reactor for CGW treatment. The discoveries in this study will facilitate successful development of such bioaugmentation systems and can provide some guidance for practical bioprocesses.