In this study, biochar BC400 and BC700 were prepared, characterized and coupled with heterotrophic nitrification-aerobic denitrification (HNAD) strain Z03 for nitrogen removal experiments. The characterization results showed that BC700 has a higher specific surface area and a more complex multilayered pore structure, with increased aromatic condensation and higher crystallinity. BC400 and BC700 both have good redox activity, while BC400 has stronger electron donor capacities and BC700 owns better electron transfer properties. In addition, both BC400 and BC700 contain relatively high levels of dissolved organic carbon (DOC), reaching at 62.95 and 51.617mg/g respectively. BC400/BC700 coupled with strain Z03 can significantly improve the NH4+-N removal performance of low-temperature and low C/N wastewater compared with the control group. At a dosage of 4.0 g/L, the removal rate of NH4+-N reached to 95.16% (BC400 + Z03) and 84.37% (BC700 + Z03) within 72h, respectively. Higher than the sum of adsorption by BC400/BC700 (16.19%/18.85%) and microbial degradation (41.03%). Besides, the BC400 + BC700 + Z03 NH4+-N removal systems provide higher nitrogen removal efficiencies than BC400/BC700 + Z03 nitrogen removal systems. When the dosage (BC400 + BC700, mass ratio 5:1) reaches 3.0g/L, it can achieve more than 90% NH4+-N removal rate within 48h. The reasons for the promotion of biochar on microbial denitrification were analyzed as follows: 1) DOC can provide an additional carbon source for microorganisms; 2) biochar, as a pH buffer, can neutralize the acidity due to nitrification; 3) BC400 and BC700, as materials with good redox activity, may play a role in promoting the activity of electron transfer system and enzyme activity.