Abstract: Humic acid (HA) in the sludge affects the anaerobic fermentation process. In order to investigate the effect of HA content on the methanogenic efficiency of anaerobic fermentation, mesophilic anaerobic fermentation tests of HA content 0, 200, 300, 400 mg/L (recorded as T0, T1, T2, T3 systems) were conducted. The methane content, cumulative methane yield and production dynamics, as well as the correlation between methane yield and HA content were analyzed. The results showed that the pH in anaerobic fermentation systems with HA changed from 6.9 to 7.9. HA had buffering function on the change of pH value. The peak of methane content in T1 treatment was 66.7%, which was higher than 61.5% of T0. However, with the increase of HA content, the peak of methane content decreased to 62.0% (T2 system) and 60.0% (T3 system). The modified Gompertz equation was used to fit the cumulative methane yield and the results showed that HA inhibited the theoretical methane production of anaerobic fermentation. The inhibition efficiency was more significant with increasing HA, and the theoretical methanogenic potential of T0, T1, T2 and T3 systems were 441.1, 432.1, 397.1 and 253.5 mL, respectively. The correlation analysis showed that the HA content and cumulative methane yield were negatively correlated. The cumulative methane yield gradually decreased with increasing HA. Therefore, HA inhibited methane production during sludge anaerobic fermentation, and the peak of methane content in the biogas increased when the HA content was low.