It is efficient to extract active components from traditional Chinese medicine (TCM) by using ultrasonic wave. There has been many studies of the strength, frequency and action time of ultrasonic wave influencing extract active components from TCM. There is no report about the research of another parameter-ultrasonic action on-off ratio which influences the process of extrating effective composition from TCM. Using ultrasonic wave to extract resveratrol from the Polygonum cuspidatun sieb, the influence of ultrasonic on-off action ratio on the purity of resveratrol extracts from the Polygonum cuspidatum sieb is compared by analyzing the resveratrol with high performance liquid chromatography. The extracts from herbs under different ultrasonic action conditions have different impurities. The impurity of the extracts is increasing along with decreasing of on_off ratio of ultrasonic.