Experiments on organic acids conversion in methanogenic-phase of the two-phase anaerobic process were conducted. The results showed that when the acidogenic-phase was in mixed acids fermentation and its load was 41.5 kg/(m3-d), the load of methanogenic-phase was 6.05 kg/(m3.d), the substrate conversion velocity of bacteria inhabited at the same height of UASB reactor as follows: acetic acid > ethanol > butyric acid > propionic acid; ethanol-utilized microbial had the same pH range as methanogenic bacteria; ethanol-type fermentation was the optimal acidogenci-type fermentation for the two-phase anaerobic process; the conversion of acetic acid was rather high, but the others organic acids will be convert to acetic acid make it the rate-limiting step for anaerobic degradation.