Abstract: Micro-aerobic and anaerobic hydrolysis acidifications of petrochemical wastewater were conducted and compared to investigate the influence of micro-aerobic condition on the high salt content and refractory industrial wastewater. The dissolved oxygen (DO) of micro-aerobic condition is about 0.5 mg/L, and the oxidation reduction potential (ORP) of anaerobic reactor is below -300 mV. The results showed that the average removal rate of COD_Cr with micro-aerobic hydrolysis acidification was 25.0%, while the average removal rate of COD_Cr with the anaerobic hydrolysis acidification was 23.5%. The rate of UV₂₅₄/COD_Cr with micro-aerobic hydrolysis acidification was higher than that of anaerobic hydrolysis acidification, indicating a higher removal of aromatic and conjugated double bond organics in micro-aerobic condition than in anaerobic condition. The acid production and acidification rate was higher in anaerobic condition than in micro-aerobic condition because part of produced acid could be used by heterotrophic bacteria simultaneously during the hydrolysis acidification. The SO₄^2- in anaerobic reactor reduced obviously, while it did not decrease in micro-aerobic reactor because the activity of sulfate reducing bacteria was restrained by oxygen. Therefore, the micro-aerobic condition can decrease the production and emission of H₂S. Micro-aerobic hydrolysis acidification is more suitable for high salt content and refractory petrochemical wastewater treatment if DO can be controlled properly.