Enterohemorrhagic Escherichia coli O157:H7 (EHEC O157:H7) is zoonotic pathogen linked with severe human illnesses. In order to evaluate risks of the pathogen in wastewater treatment process, the occurrence and removal of them were studied in four municipal wastewater treatment systems every three months from November, 2011 to July, 2012. Real-time PCR for E.coli O157:H7 was performed on extracted DNA using primers targeting eaeA gene. The method was used to detect E.coli O157:H7 in raw wastewater, mixed liquor, secondary clarifier effluent, excess sludge and sludge liquor of the four systems mentioned above. The quantity of E.coli O157:H7 in raw wastewater varies with the season, ranging from 10³ to 10⁵ copies per milliliter(copies/ml) in summer, from 10² to 10³ copies/ml in winter. However, effluent keeps relatively steady, 10 to 10² copies/ml. The removal of E.coli O157:H7 was about 1 to 3 orders of magnitude in copies per milliliter. The amount in sludge liquor is more close to influent than in effluent, which showed the sludge liquor should not be discharged into natural water bodies directly as it still threatens people’s health. The result also indicated that EHEC might not be a kind of bacteria joining to make up zoogloea. In order to understand the mechanism of the removal of EHEC, the quantity of E.coli O157:H7 in effluent in copies per millimeter was compared with that of excess sludge, and the latter makes up a proportion ranged from 54.22% to 96.14%, which means the majority of E.coli O157:H7 came into returned sludge in secondary sedimentation tank. Thus, temporary transference from water phase to sludge is one reason of E.coli O157:H7 decreasing.