The micro aerobic and anaerobic hydrolysis acidification were used to improve the quality of gelatin wastewater. The effects of hydraulic retention time were explored. The experimental results showed that when the hydraulic retention time was 72 h, the COD removal rate of micro aerobic reactor could reach up to 25% when dissolved oxygen was 1.3~1.6 mg/L, and that of anaerobic reactor up to 22% when dissolved oxygen was 0.3~0.5 mg/L. The content of VFA of micro aerobic reactor was 12 mg/L, and that of the anaerobic reactor was lower to 8 mg/L. The pH of aerobic reactor was reduced to 7.5 from original 12.5, and that of the anaerobic reactor was reduced to 8.0. The removal rate of protein for both reactors was more than 90%. However, the ammonia nitrogen of micro aerobic reactor was only 22 mg/L, less than that of the anaerobic reactor, which indicated that anaerobic conditions was beneficial for spread and volatile of ammonia nitrogen. All the data suggested that hydrolytic acidification of micro aerobic reactor was better than anaerobic reactor for the pretreatment of gelatin wastewater.