The Fe-0/GAC mixed-potential corrosion reactor was used to treat the complicated, toxic, and refractory ABS resin wastewater. In the 100 days continuous run, the effect of the packing particles passivation on the treatment efficiency of the Fe-0/GAC mixed-potential corrosion reactor was investigated seriously. The formation mechanism of the compounds in passive film was investigated first by SEM, EDS, and X-ray dot-mapping, which was the precondition for the control of the passivation of packing particles. The results show that the passive film consisted of five kinds of compounds such as Fe-3(PO)(2)center dot 8H(2)O, FePO4 center dot 3H(2)O, Fe2O3, Fe3O4, and FeS, which obstructed the formation of macroscopic galvanic cells between sponge iron (Fe-0) and GAC and decreased the COD treatment efficiency of the Fe-0/GAC mixed-potential corrosion reactor from 45 to 55% to 0%. The formation of passive film mainly resulted from the elements of S and P, which were from the SO42- and PO43- in ABS resin wastewater. Therefore, the inorganic ions in wastewater, especially for SO42- and PO43-, should be removed first before the treatment of the Fe-0/GAC mixed-potential corrosion reactor.