In this paper, the application of the ultrasonic technology to the enzyme desizing pretreatment of polylactic acid (PLA) fiber was studied, and the different effect of such technology and traditional treatment on the PLA properties was also discussed. The results showed that, with the aid of the ultrasonic wave, the mass decrease rate and the whiteness of fiber increased 15% and 25% respectively, while the tear strength decreased nearly 6% after the enzyme pretreatment. Besides, the time of ultrasonic pretreatment also decreased 20% compared with traditional process. The optimal enzyme desizing conditions and fomula were accomplished, the concentration of H2O2 and refining agent should be 6 mL/L and 4 g/L, the recommended enzyme concentration was 5 g/L.