Modified anaerobic-anoxic-aerobic (A2O) technology for dye wastewater was studied. The effects of Hydraulic retention times (HRT), sludge retention time (SRT) and carbon source on treatment efficiency were discussed. Scanning electron microscopes (SEM) for different sludge and UV-visible spectrum analysis for influent and effluent were investigated respectively. The results showed that when COD of influent was 600~1200 mg/L and the dye concentration was 20~40 mg/L, HRT had the obvious effect on treatment efficiency of modified A2O system. Total treatment efficiency of the improved system was more than 90% at the optimal HRT (29 h). The dye structure was demonstrated to be changed during the process. Finally, the dye and its intermediates were transformed and mineralized respectively.