Aerobic denitrification bacteria are currently being explored as promising microorganisms with which to treat wastewater containing concentrated ammonium and organic carbon at psychrophilic temperatures. However, little is know regarding the mixed culture of these microbes under low-temperature shocks. In this study, a lab-scale aerobic denitrifying process was established in a sequencing bioreactor at 25°C, and successfully performed in a short-term temperature downshift period that ranged from 25°C to 5°C. The specific aerobic denitrification rate was maximized at 15°C [15.33 mg N/(g SS h)], and this process effectively removed chemical oxygen demand (COD) and nitrate (above 90%) even at 5°C. The results of Illumina MiSeq 16S rRNA amplicon analysis revealed a relatively stable microbial community across the transitional periods of temperature. Pseudomonas was the most abuntant genus below 15°C, and the amount of species Pseudomonas alcaligenes significantly increased at 5°C with potassium nitrate as the primary nitrogen source. The communities of aerobic denitrifying bacteria in the temperature downshift period were all highly organized functionally, and they were relatively stably across the temperature gradient. These results will enhance understanding regarding the microbial ecology of aerobic denitrifiers in mixed cultures, leading to a more effective controlling of aerobic denitrification process during temperature transition states.