Abstract: The oxidation of free hydroxyl radicals (·OH) is one of the most important mechanisms for the electrolysis oxidation technology. The generation mechanism of producing ·OH in periodically reversing electrolysis process was investigated in the 0.015 mol/L Na2SO4 solution and the detective methods of ·OH were selected. Furthermore, the influence factors of the generation volume of ·OH were studied. The results showed that colorimetric analysis using dimethyl sulfoxide (DMSO) as the trapping agent was superior to that using salicylic acid as the trapping agent. The maximum amount of free hydroxyl radicals was detected when the volume of DMSO (200 mmol/L) was 50 mL, with the reaction voltage of 8?V, exchange period of 10 s and reaction time of 90 min. The cumulated amount of the free hydroxyl radicals by adding H2O2 increased more than that by adding H2SO4, indicating that the periodic reversing electrolysis method combined with adding H2O2 could be widely used in field of wastewater treatment.