The kinetics of sulfite oxidation in the magnesium-based wet flue gas desulfurization (FGD) process were investigated in a stirred bubbling reactor by varying the concentrations of MgSO3 and MgSO4, pH, air flow and temperature. The reaction was found to be 0.88 order with respect to magnesium sulfite and the oxidation could reach the maximum rate when the pH value was close to 6.5. The mechanism of the oxidation was discussed, and it was concluded that the oxidation is controlled by diffusion of oxygen. These results would be useful for the design or process optimization of the magnesium-based wet FGD system.