The emission of polychlorinated dibenzo-p-dioxins and -furans (PCDD/Fs) is a serious concern from municipal solid waste incineration (MSWI). The utilization of inhibitor is an effective method to inhibit the formation of PCDD/Fs. In the present study, the thiourea is used as an effective inhibitor in two full-scale MSW incinerators, and the suppression mechanisms are investigated in detail. After the injection of thiourea, the total PCDD/Fs emission factor decreases from 6.19 and 7.89 mg/t MSW (0.13 and 0.18 mg I-TEQ/t MSW) to 1.35 and 1.34 mg/t MSW (0.029 and 0.026 mg I-TEQ/t MSW), respectively for incinerator A and B. In the post-combustion zone, the contribution of three pathways on PCDD/Fs formation is found to be in the order of de novo synthesis > chlorophenol (CP) route formation > chlorination of dibenzofuran (DF) and dibenzodioxin (DD). The S and N in thiourea poison the catalytic metals, thus significantly hindering the de novo synthesis of PCDD/Fs. Since metal catalysts for precursor-formation are not as crucial as for de novo synthesis, the thiourea has less suppression effect on CP-route synthesis, which is identified from the reduction of signal intensity of CP-route congeners. In addition, the chlorination of DF/DD is also observed to be suppressed, indicating the transformation of Cl-2 to less activate HCl caused by thiourea. This study provides a robust basis to promote the industrial application of inhibitor to control the emissions of PCDD/Fs from MSW incinerator.