Bioremediation is an efficient and popular approach for domestic wastewater treatment while the pollutant discharge standards are difficult to achieve under low-temperature conditions. The application of cold-resistant bacteria has gained increasing attention, but direct introduction to sewage leads to poor environmental adaptability and low microbial activity. Biochar was used as a carrier to immobilize the bacteria to improve microbial survival and activity in this study. The basic physicochemical properties of bacteria immobilized by biochar and ammonium nitrogen removal efficiency were analyzed. The process mechanism of ammonium nitrogen removal was further explored using kinetic fitting and molecular simulation calculations. The results showed that biochar immobilization of cold-resistant bacteria achieved a significantly higher ammonium nitrogen removal rate of 0.88 mg/(Lh) compared to free mixed bacteria (0.74 mg/(Lh)) and biochar alone (0.22 mg/(Lh)). It also exhibited a removal efficiency of 96.56%, which was 15.02% and 72.58% higher than that of free mixed bacteria and biochar, respectively. Adsorption kinetics further revealed that the pseudo-second-order kinetic equation was a better fit for characterizing ammonia-nitrogen removal by biochar-immobilized cold-resistant bacteria. Combining microscopic morphology analysis and molecular simulations demonstrated that enriching functional groups on biochar enhanced its NH4+ adsorption capacity by increasing surface activity and polarity, as well as the biodegradation ability of NH4+ by improving the interactions between biochar and active enzymes. These findings provide valuable insights into developing more effective ways to improve wastewater treatment efficiency under low temperatures.