Apocynum venetum (AV) fibers were extracted by the combination of low (28 kHz) and high frequency (53 kHz) ultrasonic treatment after aqueous alkali maceration. The surface impurities and cementing components between fibers in the range of 10-50 ${mu}m$ were removed by low frequency ultrasound. The surface impurities in the range of 2-8 ${mu}m$, as well as the residuals in the surface depression and inner cavum of fibers were further eliminated by high frequency ultrasonic irradiation. The treatment did not change crystal structure of cellulose I of AV fibers and could lead to a higher degree of crystallinity. Meanwhile, the examination of mechanical properties showed that the AV fibers could be used for textile industry. It is demonstrated that the combination of low and high frequency ultrasound after alkali treatment is simpler, more controllable and more environment-friendly and is a promising degumming method for textile industry.