Abstract: A gradient acclimation of anaerobic sludge in an four-chamber anaerobic baffled reactor (ABR) was determined. Glucose, a mixture of glucose and phenol and complete phenol were concurrently added to ABR. The changes of COD, phenol and pH in each chamber of ABR were analyzed and the microbial community structure was investigated by the Illumina high-throughput sequencing technology. The results showed the greatest removal rate of COD and phenol was observed in the first two chambers. The treatment efficencies of the ABR was not fully achieved because COD in the third and fourth chamber was always maintained at 100 mg·L-1. The pH values in the first chamber of ABR varied from 5.4 to 6.8 when glucose was used as the solo substrate. The pH values in the front chambers increased when phenol was added. Howerver a decrease of pH in the fourth chamber indicated there were acid substances contained in outflows. The abundance of Firmicutes which was the dominant species in the bacterial community decreased from 66.5% to 15.2% and Desulfobacterota became the dominant species. The abundance of Methanobacteriaceae which was the dominant species in archaeal community decreased by 90%. The abundance of unassigned archaea decreased from 15% to 0. Woesearchaeales and Micrarchaeales became the dominant species of archaeal community. The result of Redundancy Analysis (RDA) showed the concentration COD and phenol were the major environmental factors influencing the abundance of microbe. The distribution of Lokiarchaeia and Asgardarchaeota were correlated with some unknown environmental factors.