The current study aimed to determine the effects of NH4+ on anaerobic digestion (AD) metabolism and the feasibility of using NH4HCO3 to improve methane production in an AD system when treating a low-C/N-ratio food waste (FW). Increasing the ammonium concentration (500–1000 mg NH4Cl-N/L) added into the AD system did not limit the methane production but caused the volatile fatty acid (VFA) accumulation, forming an “inhibited steady-state” system. The addition of 200 mg NH4HCO3-N/L increased methane yield by 20% by aiding the microbial oxidation of VFAs. The high acetate content (65–85%) and abundance of acetoclastic methanogens (Methanosaeta and Methanosarcina) indicated an efficient acetoclastic methanogenesis process, which was facilitated by NH4HCO3. The long-term operation of the AD system demonstrated that NH4HCO3, at a concentration of 200 mg N/L, was capable of forming an active buffer system with NH4+ and VFAs, enhancing methane production (221 ± 86 mL/g VS).