Display omitted •An efficient HSND-MBBR was established successfully by inoculating HNAD bacteria.•The HNAD via shortcut process dominated in HSND-MBBR and enhanced at low C/N.•HNADs (S. maltophilia), DPAOs and GAOs were the main function genus.•HSND-MBBR could regulate nitrogen metabolic pathways to adapt various C/N ratios. Traditional simultaneous nitrification and denitrification process was restricted by the stringent operation conditions and the inhibition of heterotrophic bacteria to ammonia oxidizing bacteria. To address these problems, a long-term simultaneous nitrification and denitrification-moving bed biofilm reactor (SND-MBBR) was successfully started by inoculating heterotrophic nitrification and aerobic denitrification (HNAD) bacterium Stenotrophomonas maltophilia DQ01 and investigated at different C/N ratios. Remarkable SND efficiency (94.21%) and total nitrogen removal (94.43%) were achieved at C/N of 7.5, and declined with C/N decreasing due to the reduction of electron donation and consumption. The combined stoichiometry and kinetics analyses confirmed that the HNAD via short-cut process dominated in feast phase and spiraled upwards with decreasing C/N, as evidenced by extremely low NXR activity and Nitrospira abundance (below 0.1%), suggesting that the inadequate electron donation was favorable for partial nitrification and denitrification. Furthermore, endogenous nitrification and denitrification occurred without COD consumption in famine stage, and decelerated at lower C/N due to the less stored internal carbon. High throughput sequencing revealed that HNADs (represented by Stenotrophomonas maltophilia), denitrifying phosphorus accumulating organisms and denitrification glycogen accumulating organisms dominated during the community succession and nitrogen removal process. Moreover, Candidatus Competibacter and Dechloromonas were negatively correlated with nitrite, leading to the accumulation of nitrite in famine phase. COG analysis showed the accumulated nitrite might affect the defense system and signal transduction. This study provided a new strategy for nitrogen removal and gave a new insight into the SND mechanism under different C/N conditions.