Coal gasification wastewater is difficult to treat by conventional biological processes due to the high load of recalcitrant organic matter and high toxicity. In this study, a growth promoting agents and bacterial consortia were used to enhance the treatment efficiency of secondary effluent from coal gasification wastewater, and the microbial mechanism of the bioaugmentation was discussed. The results showed that an addition of crude oil degrading-bacteria has a significant positive effect on chemical oxygen demand (COD) removal from wastewater, in which the average COD removal rate during the first three cycles was increased by 19.7%, compared with the control group without bioaugmentation. In contrast, the addition of growth promoting agents can significantly reduce the COD concentration during the first cycle, but the strengthening effects is difficult to maintain for a long time. The average COD removal rate was increased by 21.5% by supplementing both crude oil-degrading bacteria and Tween 80 during the six cycles, compared with the control group without bioaugmentation. It showed that the combined addition of growth promoting agents and bacterial consortia has good stability and sustainability. Furthermore, a high-throughput sequencing analysis showed that the addition of growth promoting agents and bacterial consortia has little effect on dominant genus (such as Candidatus Competibacter and Defluviimona), but significantly increased the diversity of microbial communities. Limnobacter and Gaiella with high weight and closeness centrality are the keystone taxa in the coal gasification wastewater treatment system. Meanwhile, super pathways of vanillin and vanillate degradation are the key degradation pathways of microorganisms, indicating that the vanillin and vanillate may be the common degradation intermediates or their structural analogues of a variety of complex organic compounds in coal gasification wastewater.