Combined water washing with catalysis was applied in pyrolysis process to enhance the destruction efficiencies of PCDD/Fs and dl-PCBs in MWI fly ash. The results indicate that water or carbonated water washing effectively reduces chloride content in fly ash but it contributes minor enhancement of PCDD/Fs and dl-PCBs destructions achieved with pyrolysis process. Being pyrolyzed at temperature of 350 °C for 1 h, the removal efficiencies of 96.7%, 98.5% and 96.2% for PCDD/Fs, and 79.3%, 92.3% and 87.3% for dl-PCBs are achieved for unwashed fly ash, water washed fly ash and carbonated water washed fly ash, respectively. Nonetheless, TEQ concentrations of PCDD/Fs and dl-PCBs in pyrolyzed fly ash remain higher than the European end-of-waste criteria (20 pg WHO-TEQ/g). As palladium was added to the system as catalyst, the decay rates of PCDDs, PCDFs and dl-PCBS in fly ash increase significantly even with a much shorter pyrolysis time (15 min). Increases of PCDD/Fs and dl-PCBs destruction efficiencies by pyrolysis combined with catalysis could be ascribed to the inhibition of precursor formation at 200–300 °C by Pd. Synergistic effect of water washing and catalysis in pyrolysis process at 350 °C reduces the concentrations PCDD/Fs and dl-PCBs to lower than 20 pg WHO-TEQ/g, enabling the reuse of MSW fly ash.