The influence of ultrasound energy densities on dewaterability of drinking water treatment sludge and the involved influencing factors was investigated. The dewaterability of sonicated sludge was improved at relatively low energy densities of 0.03 and 1.0W/mL in less than 10minutes. However, at relatively high energy densities, the dewaterability of sludge was deteriorated regardless of ultra-sonication time. The content of solubilized organic matter in the supernatant of sonicated sludge was increased. Additionally, the floc characteristics changed obviously and the dewaterability got worse as the energy density increased. At lower energy densities of 0.03 and 1.0W/mL, the dewaterability of sludge was not correlated with solubilized organic matters and floc characteristics of sonicated sludge. However, at a high energy density of 5W/mL, the dewaterability was significantly related to solubilized organic matters and Zeta potential of sludge.