The work aims to study the physiological mechanism of ultrasonic treatment on enzymatic browning in view of the oxidative browning of fresh-cut sweet potato and find the optimal ultrasonic treatment time. Fresh-cut sweet potato slices were treated with ultrasound for 0, 5, 10 and 20 min at frequency of 40 kHz and temperature of 25 ℃. The color, malondialdehyde content, browning related enzyme activities and total phenol content of fresh-cut sweet potato at 4 ℃ storage were tested. Moderate ultrasonic time effectively maintained the brightness and color of fresh-cut sweet potato, inhibited the accumulation of malondialdehyde, passivated the activity of polyphenol oxidase and peroxidase, stimulated the activity of phenylalanine ammonia-lyase, improved the total phenol content and antioxidant capacity, and ultimately delayed browning of fresh-cut sweet potato. The best anti-browning effect is achieved by ultrasonic treatment for 10 min, and ultrasonic treatment for 20 min will cause damage to sweet potato tissue and aggravate browning.