In this paper, the effect of heavy metals on the ability of Effective Microorganisms(EM) purifying domestic sewage was studied by adding 7 kinds of solutions containing different contents of Cu 2+ ,Zn 2+ ,Pb 2+ ,As 3+ ,Cr 6+ ,Cd 2+ and Hg 2+ to sewage.The results showed that every kind of heavy metal could poison EM ,and the higher the concentration of heavy metals in domestic sewage, the more serious the poisoning. When other conditions were given, the removal rate of CODcr was gradually reduced with the increase of heavy metals' concentration...