Abstract: PDS desulfurization method has been applied widely because of effectively solving the problem of hydrogen cyanide poisoning. However, the by-products problem is still a hotspot concerned in whole desulfurization field at present. The reaction between binuclear sulfonated phthalocyanine cobalt (bi-CoPc) and sodium sulfide was studied by the cyclic voltammetry in this paper. The results showed that the oxidation reaction of Na₂S and Na₂S_x on glassy carbon electrode was an irreversible process controlled by concentration diffusion. And the transformation from S^2- to S₈ was not a one-step process, but firstly transformed into the intermediate, S_x^2-, then further generated S₈. Moreover, it was deduced that only 20% of S^2- involved in the reaction was converted into S₈. S_x^2-had two main existing forms within the range from S₂^2- to S₉^2-, and their forms might have influences on the formation rate and production amount of S₈. Bi-CoPc could be more readily form the coordination complex with OH^- in aqueous solution than in carbonate buffer system, but the redox reaction of Co⁺/Co²⁺ is easier in carbonate buffer system than in aqueous system. Meanwhile, S₂^2- was formed firstly in the reaction between bi-CoPc and Na₂S, S₂^2-then could transformed into S_x^2-with a longer chain, finally formed S₈. It was helpful for us to understand the mechanism of PDS desulfurization, and it also had guiding significance for reducing the formation of by-products.