The changes of sludge disintegration degree, microbial activity and particle size were examined synchronously, and the relationships between the changes and the effect of ultrasonic parameters on microbial activity were also analyzed. The results show that the process of ultrasonic sludge treatment can be divided into two stages: at the first stage, sludge flocs are disrupted into small micro-floc aggregates with the diameter of 10-20 microm, the extracellular organic substances are dissolved, bacteria are dissociated from flocs, and the microbial activity is enhanced; at the second stage, the micro-floc aggregates are disintegrated further into bacteria bodies with the diameter less than 10 microm, the intracellular organic substances are released, and the microbial activity are decreased. The two stages are not entirely separated because sludge is inhomogeneity. When sludge disintegration degree is lower than 20%, sludge floc disruption is the main effect and the microbial activity can increase over 20%; when sludge disintegration degree is between 20%-40%, some bacteria are hurt and the microbial activity can increase by less than 20%; when sludge disintegration degree is over 40%, most bacteria are damaged and sludge activity would decrease quickly. Because low energy ultrasonic can disrupt sludge floc with no damage to microorganism in a long treatment duration time, it is suitable to stimulate sludge microbial activity.