Electro-peroxone process utilizes the in-situ generated H2O2 by electro-reduction of O-2 to react with ozone molecule to produce hydroxyl radical, and achieves high efficiency in the wastewater treatment. However, detailed reaction mechanism in this process is still deficient. In this paper, a three-electrode system was adopted in the electro-peroxone process, and the origin of the synergy was revealed with the aid of the rotating ring disk electrodes. It was found that besides the main reaction between O-3 and H2O2, O-3 is also electro-reduced to O-3(center dot-) and then transformed into (OH)-O-center dot. Meanwhile, O-3 electro-reduction on the cathode can also possibly generate O-2 and promote H2O2 production. Higher concentration of OH- near the cathode originated from O-2 and O-3 reduction may also react with O-3 to produce H2O2. These two pathways contributed to the higher amount of H2O2 than the system with O-2 bubbling only, and ultimately contributed to produce more (OH)-O-center dot. All these reactions are synthetically responsible for the higher degradation rate of sodium oxalate in the electro-peroxone process than those in O-3-electrolysis and ozonation processes, respectively. Though this system was quite different from normal electro-peroxone process, the conclusions are useful for understanding the practical reactions possibly happened on electrodes in the electro-peoxone process.