Using a stirred bubbling reactor, the kinetics of oxidation inhibition of MgSO3 from a magnesium-based wet flue gas desulphurization process were investigated. The reaction was found to be 0.7 order with respect to magnesium sulfite and -0.3 order with respect to sodium thiosulfate. The pH value should be controlled at a lower level in order to hamper the oxidation of sulfite. The apparent activation energy of the reaction was calculated to be 8.68 kJ/mol and a kinetic model has been established. Pilot tests showed that sodium thiosulfate can effectively inhibit the oxidation of magnesium sulfite in the magnesium-based wet flue gas desulphurization process.