Abstract: Abstract: To solve the inhibition issue caused by volatile fatty acids in two-stage dry anaerobic co-digestion of food waste and distillers grains, microzyme was added into the reactors in the acidification process with different pre-treated times of 12 h, 24 h and 48 h respectively. After pre-fermentation, all groups underwent anaerobic digestion under the same experimental condition. The purpose was to investigate the effects of pre-treated time on methane yield, the changes of parameters such as pH, TVFA, acetic acid, propanoic acid, and ethanol concentration in the methane fermentation process, and to compare them with the control group. The most innovative idea is that of the ethanol pre-fermentation process, food waste was converted into ethanol which decreased the other volatile fatty acids` concentration in the meantime. Since ethanol is neutral and it can convert into acetic acid, which can be directly used by methanogens, this ethanol pre-fermentation process can indeed improve digestion efficiency and methane yield in the following anaerobic system. Moreover, with less volatile fatty acids produced at the beginning of digestion, the acid accumulation issue could be well relieved in the future. This meaningful topic has been focused on by our research team since 2011. Please find more detailed information in our articles if you are interested. According to this experiment, the result showed that the ranks of ethanol concentration and pH (from high to low) of all groups were: pre-treated 48h > 24h > 12h > control. However, the ranks of acetic acid and TVFA concentration were the opposite. This phenomenon is caused by the ethanol pre-fermentation which can force more glucose to degrade into ethanol instead of other volatile fatty acids such as propanoic acid. Besides, the ethanol is neutral, and it can convert into acetic acid more easily than propanoic acid. Therefore, the pre-fermentation process can overcome the acidification issue caused by the accumulation of VFAs. As compared with the control group, the accumulated methane yields of pre-treated groups with 12h, 24h, and 48h were 9.1%, 31.6% and 19.9% higher respectively. The results indicated that the pre-treated group of 24h was the most effective. As well known, methanogens` activity is very sensitive regarding tiny changes of propanoic acid concentration. The ratio of PC/AC is also a warning indicator in anaerobic system. One relevant literature shows that a range of 0~0.08 has a positive effect on methanogens. Interestingly, we found that there was a difference in the range-width among different groups. The rank of range-width (from high to low) in this result was: pre-treated 24 h>48 h>12 h> control. This result indicated that pre-fermentation could increase the suitable range-width in order to prevent the digestion system from acid accumulation issue. The same situation happened with the relationship between methane yield and initial ethanol concentration. It deeply explained the reason why anaerobic digestion could benefit from pre-fermentation. From another point of view, both the concentration of acetic acid and ethanol, microzyme and methanogen were reaching a dynamic balance. However, this complex mechanism of biology and dynamics should be discussed in future research.