Use the water / ethanol as the reaction medium, benzaldehyde as raw material, thiamine hydrochloride （VB1） as catalyst,benzoin was prepared in alkaline solution by ultrasonic cavitation and phase transfer catalysis . The effects of various reaction conditions, such as ratio of raw materials, reaction temperature, reaction time, system pH, the type of phase transfer catalyst,were studied in detail.The best conditions were in ethanol / water mixed solution,the amount of benzaldehyde was 5 mL ，the catalyst thiamine hydrochloride was 2.0 g, the quality of phase transfer catalyst was 10% ~ 12% of benzaldehyde ,the volume of disti lled water was 2 mL, ethanol was 8 mL,the system pH was 9.3~9.7,reaction temperature was about 70 ℃,the reaction time was  90 minutes, the ultrasonic frequency was 80%.The yield reached 97.32%, the product content was 90%.The process had the advantagements,such as mild reaction condition and higher conversion rate etc.