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Pyrite is an important gold carrier during gold leaching, but it is readily oxidized, which causes environmental problems such as acidic mine drainage. Thus, it is necessary to study the oxidation mechanism of pyrite in a gold-leaching electrolyte. The surface oxidation reaction of pyrite is a multi-electron transfer electrochemical oxidation process. Based on this, electrochemical technology was used to explore the electrochemical oxidation mechanism of pyrite in a Cu2+-Cit3−-S2O32−system. The surface oxidation products of the pyrite were characterized by Raman spectroscopy and X-ray photoelectron spectroscopy. The experiments showed that the thiosulfate concentration did not change the oxidation mechanism of pyrite at 0.30 V under the conditions of this experiment. Increasing the concentration and potential of thiosulfate accelerated the oxidation rate of pyrite. The electrochemical oxidation of pyrite in the Cu2+-Cit3−-S2O32−system occurred in two stages. At low potentials in a passivated state, the process was diffusion-controlled, and the surface oxidation rate was slow. When the potential exceeded 0.50 V, the passivation film on the surface was penetrated, allowing the oxidation reaction on the surface of pyrite to continue. The results of this experiment are useful for deepening the understanding of the oxidation mechanism of pyrite in electrolytes.