Four sequential batch reactors (SBRs) containing synthetic sewage for denitrification were investigated in this study. Three of them had added one of the three redox mediators, which were anthraquinone-1,5-disulfonate (AQDS), 1,2-naphthoquinone-4-sulfonate (NQS), and 2-hydroxy-1,4-naphthoquinone (LAW), operated at 20 °C and 10 °C, and an additional one to serve as the control. Results showed that 10 °C inhibited denitrification to a considerable extent, but the addition of mediators increased the denitrification rate and efficiency. The total nitrogen removal efficiency increased in the presence of three different redox mediators (100 μmol/L), among which LAW express the best accelerating effectiveness at normal temperature and NQS at low temperature. This may be due to the growth of microorganisms, whose community compositions changed considerably when the different redox mediators were added. Therefore, Illumina MiSeq sequencing was used to identify the different microbial communities. Thauera was dominant at 10 °C (25.60%). Furthermore, the addition of mediators greatly promoted Thauera growth (31.11%-42.41%), especially LAW (42.41%). At 20 °C, Candidatus Competibacter (8.31%-9.59%) and Denitratisoma (6.33%-7.39%) were dominant. Thauera and Denitratisoma are denitrifiers. These results could improve understanding of the sewage biological process at low temperature.