To extract As bound to amorphous and crystalline iron oxides, this study proposed simultaneous application of oxalic acid and dithionite, which was observed to induce synergistic effect and accomplish effective extraction of As bound to both iron oxides. However, the formation of arsenic sulfide decreased overall removal of As because the insoluble precipitate form of As remained as a residual fraction of As in soil. Therefore, stepwise addition of dithionite in the simultaneous application was applied to minimize the formation of secondary minerals and maximize the As extraction. As a result, 74% of As bound to amorphous iron oxides and 65% of As bound to crystalline iron oxides were removed. More importantly, the stepwise application of oxalic acid and dithionite was effective to reduce the bioaccessible concentration of As in the treated soil. Therefore, the proposed application could reduce the potential risk of contaminated soil to human health by extraction-based remedial action.