By experiment, the coagulation effect, influence factors of the backwashing wastewater in the groundwater works for iron and manganese removal have been investigated, and its coagulation mechanism has been discussed. The results show that the CPF and PAM coagulation effect is the best, the ferric chloride is the second best, the PAC is worse, and the aluminum sulfate is the worst. When the iron content of the backwashing wastewater is about 100 mg/L, the natural settling effect is not good, but if some coagulant is added to the backwashing wastewater, the backwashing wastewater has good sedimentation effect in a short time. In the initial 10 - 30 min after the coagulation, the iron content of the clear supernatant is lowered quickly, and with the increase of the sedimentation time, its iron content decreases slowly. When pH value is about 7.5, the coagulation mechanism of the aluminum sulfate, polychlorinated aluminum and ferric chloride may mainly have electrical neutralization and adsorption bridging action, and the PAM coagulation mechanism may be mainly adsorption bridging action, and the CPF may have some electrical neutralization besides adsorption bridging action.