The current study aimed to determine the effects of NH on anaerobic digestion (AD) metabolism and the feasibility of using NHHCO 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 NHCl-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 NHHCO-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 NHHCO. The long-term operation of the AD system demonstrated that NHHCO, at a concentration of 200 mg N/L, was capable of forming an active buffer system with NH and VFAs, enhancing methane production (221 ± 86 mL/g VS).