To study the effect of Fe₂(SO₄)₃ as flocculant on microbial activity of activated sludge, Fe₂(SO₄)₃ in dosages of 20,40,60,80,100 mg·L-1 was dropped into ASS and four hours latter DHA, R_SOU and EPS content were examined as well as COD et al. The results showed that DHA, R_SOU and EPS content were insensitive to Fe₂(SO₄)₃ in range of 20-60 mg·L-1, and the removal rates of COD, TP and UV₂₅₄ in effluent increased greatly with the concentration of Fe₂(SO₄)₃increased. When the concentration of Fe₂(SO₄)₃ was 80 mg·L-1, DHA, R_SOU and EPS content decreased distinctly. When the concentration of Fe₂(SO₄)₃ was 100 mg·L-1, DHA and R_SOUwere inhibited further, whereas total EPS content increased greatly. Meanwhile the removal rates of COD, TP and UV₂₅₄ in effluent increased slowly, and the removal rates of SCOD and NH₃-N tended to decline.