Abstract: Sludge flocs tightly wrapping a large amount of bound water results in poor performance of sludge dewatering. The ultrasonic treatment is one of the effective methods to improve performance of sludge dewatering. The effects of ultrasonic time on performance of sludge dewatering are studied. The ultrasonic waves generate instantaneous high temperature and pressure in the liquid phase, which the acoustic cavitation and hydraulic shear effects can break down sludge flocs, release bound water that is difficult to remove, and improve performance of sludge dewatering. At the same time, it also releases a large amount of organic acids or carbonates in the cells, which changes the pH value of the sludge. However, there is an optimal action time for ultrasound. If the ultrasound action time is infinitely extended, the sludge flocs and microbial cells will become more fragmented, and the sludge dewatering effects will deteriorate.