It is difficult to solve the problem of the degradation of organic pollutants in fracturing flow-back fluid and the treatment process is complex. In this paper，the law and mechanism of ultrasonic degradation of hydroxypropyl guar gum，which is a key contaminant in fracturing flow-back fluid，was analyzed. The results suggested that the mechanism of ultrasonic degradation of hydroxypropyl guar gum was mainly physical shearing，ultrasonic mode and power were the main controlling parameters of hydroxypropyl guar gum degradation reaction，followed by concentration. The ultrasonic time，temperature，pH value and other factors had little influence on hydroxypropyl guar gum degradation reaction. For a certain concentration of hydroxypropyl guar gum solution，the optimal degradation parameters were as follows，the duty ratio was 40%，the ultrasonic power was 360 W，the ultrasonic time was 60 min and the temperature was 25 ℃. Because temperature and pH had little influence on hydroxypropyl guar gum degradation reaction，the ultrasonic degradation technology could be applied to a wide temperature range of 5—50 ℃ and pH range of 5—11.