Acetylsalicylic acid was synthesized with salicylic acid and acetic anhydride as raw materials,rare earth（Y³⁺,Ce³⁺,La³⁺） modified SnO₂ as catalyst assisted by ultrasound irradiation and the Y³⁺/SnO₂ catalyst showed the best catalytic perfor-mance.The effects of amount-of-substance ratio of salicylic acid to acetic anhydride,amount of catalyst,reaction time,ultrasonic power,reaction temperature and crystallization time on the synthesis of acetylsalicylic acid were investigated.The catalysts and synthetic products were characterized by FT-IR,SEM and XRD.The optimum experimental condition with the yield of acetylsalicylic acid of 83.6 % was obtained as follows：the amount of substance ratio of salicylic acid to acetic anhy-dride is 1∶2,the amount of catalyst（Y³⁺/SnO₂ ） is 8% that of salicylic acid,reaction temperature is 70 ℃,ultrasonic power is 70 W,reaction time is 30 min and crystallization time is 2 h.The catalyst can be reused for 5 times which still remains high catalytic activity.The catalyst has the advantages of good catalytic effect,no pollution,no corrosion and high reuse rate.