Solid acid catalyst SO42-/ZrO2-MgO was prepared by impregnation and was characterized by means of XRD, FTIR, NH3-TPD. The characterization results indicated that the addition of Mg in SO42-/ZrO2 was beneficial to the formation of tetragonal crystalline t-ZrO2 and increase the acid strength and acid amount. The catalyst SO42-/ZrO2-MgO was used in cellulose acetate propionate (CAP) synthesis. The catalyst containing the ZrO2/MgO molar ratio of 15/1 and being calcined at 500 ℃ showed the best catalytic activity. The yield of CAP reacted at 50℃ for 8 h was up to 116.40%.