The redox process of sulfur in soil plays an important role in the release and immobilization of arsenic (As), but the mechanism and contribution of sulfur species to the oxidation process of trivalent As are not clear. In this study, sulfite (SO2- 3) which is a ubiquitous intermediate of sulfur transformation was selected to examine its oxidation and immobilization mechanism for As(III) in soil. The results showed that sulfite significantly promoted the oxidation of As(III) in soil. Compared with CK, the oxidation efficiency of As(III) in soil suspension increased by nearly 30% within 10 h, while the oxidation of As(III) in soil solid phase extraction was more significantly, and the concentration of As(III) in adsorbed state decreased from 73.53 μg/L to 35.27μg/L. At the same time, the addition of sulfite significantly reduced the total As content in the solution and promoted the fixation of As in soil. Electron paramagnetic resonance (EPR) analysis showed that sulfite radical (SO₃^·-) was the main driving force of As(III) oxidation, and the coupling of iron ions and sulfite was the main mechanism of the formation of free radicals. In conclusion, sulfite significantly promoted the oxidation and fixation of As(III) in soil.