Buffering is requisite for anaerobic digestion (AD) to maintain suitable pH for methane production. In this study, mesophilic AD of swine manure at total solid (TS) from 4% to 14% was carried out to investigate the formation mechanism of buffer system. Results indicated that the rapid ammonia release (3.5-6.6 g-N L-1) greatly neutralized the accumulated volatile fatty acids (VFAs) (21.4-32.5 g L-1) to prevent pH from lower than 6.4. The released ammonia and accumulated VFAs established the initial stage of buffer system, ensuring suitable acidbase environment for methanogens to acclimate to high ammonia concentration at TS >= 8%. With the consumption of VFAs by acclimated methanogens, more CO2 in biogas was seized and transformed into carbonates to remedy the decreased VFAs for buffering free ammonia from overmuch pH increase. The improved methane production of 303.1 +/- 23.0 N mL g-VSadded-1 was obtained at 8% TS under the convoy of ternary buffer system. Meanwhile, the ternary pH buffer system with ammonia-carbonates-VFAs that transformed obeying ionic equilibrium was visually revealed by a simulated ternary contour during overall AD process. Such understanding of buffer system may contribute to develop more effective pH adjustment measures to counter ammonia inhibition on methane production.