In order to enhance the efficiency of anaerobic digestion, the effects of ultrasonic pretreatment, alkaline pretreatment and the combination of these two methods have been studied on sludge disintegration by using multifrequency ultrasonic batch. The results showed that the combining of ultrasonic and alkaline treatment was more effective than alkaline or ultrasonic treatment alone in releasing SCOD and VSS solubilization. The VSS reduction rate was 15.98% with ultrasonic pretreatment alone, 22.12% with alkaline pretreatment alone(NaOH/TS = 0.04). When the sludge was pretreated by the alkaline treatment (NaOH/TS = 0.04) for 24 h followed by ultrasonic vibration for 60 min, and simultaneous ultrasonic (60 min) and alkaline( NaOH/TS = 0.04) treatment, the VSS reduction rate could reach 51.45% and 54.45% respectively. Two distinct phases of hydrolysis were observed. The first phase was a very rapid increase in solubilization, followed by a much slower second phase. According to kinetic analysis for first rapid phase, the simultaneous alkaline and ultrasonic treatment could not only get the highest hydrolysis rate among these methods, but also reduce the pretreatment time in ultrasonic pretreatment and alkaline dose in alkaline treatment.