The reaction mechanism of MgO catalytic ozonation for phenol degradation was investigated by verifying their consistency with the existing three mechanisms. The effect of hydroxyl radical(·OH) inhibitor on phenol removal was discussed. At the same time, the yield of ·OH, the position of O3 and the role of phenol in the system were also studied. The results indicated that the ·OH was present in both reaction systems of O3 and MgO / O3 . The amount of ·OH in MgO / O3 system was 2.14 times of O3 system. The concentration ratios of ·OH and ozone in O3 and MgO / O3 systems were 1.47×10-9 and 3.15×10-9, respectively. Phenol played an important role in promoting the adsorption of ozone on the surface of MgO in MgO / O3 system. Ozone was decomposed into ·OH when it was adsorbed on the surface of MgO. Then, some of ·OH was released into the solution to degrade phenol. And other was kept on the surface of MgO to increase the density of surface hydroxyl.