This study investigated the acute effects of Cr(VI) shock load at 2.5, 6 and 25 mg/L on the performance and bacterial community structures in aerobiotic activated sludge reactors. The results showed that 8-day Cr(VI) toxicity made the removal rates of COD and NH3-N obviously decrease in all reactors. Furthermore, the higher Cr(VI) concentration was, the more severe influence on the whole system would be. The effect of Cr(VI) on NH3-N removal was more serious than that on COD and the function of nitrification was harder to recover. The SOUR values dropped by 73%, 68% and 31% at 2.5, 6 and 25 mg Cr(VI)/L, respectively. The data of specific oxygen uptake rate (SOUR) showed that though the concentration of Cr(VI) was low, the whole respiratory activity of bacteria was much affected. The relative abundance change of genus between initial stage and the last showed that the bacterial community structure changed significantly. Comparing with the initial stage , the phyla of Proteobacteria, Acidobacteria and Planctomycetes markedly reduced at the end stage. The genera of Ferruginibacter, Coxiella and Rhodanobacter markedly reduced. Although the performance of activated sludge can be restored at the end, the whole respiratory activity of bacteria was still at a low level according to the data of SOUR.