In this study, an online ultrasonic equipment was employed to control membrane fouling in an anaerobic membrane bioreactor for waste activated sludge digestion. Four groups of ultrasound parameters were tested for the performance of membrane fouling control. The ultrasound power intensity of 0.18W/cm2 and timing of 3min/h were considered to be optimal since that membrane fouling could be effectively controlled and no membrane damage was observed. The results of the analysis on the membrane fouling contributions and the scanning electron microscope observation of the fouled membrane indicated that the cake layer, which was the dominant membrane fouling, could be controlled effectively by ultrasound.