In this paper, ultrasonic pretreatment was used to improve the aerobic/anoxic digestion of sewage sludge. The extracellular polymeric substances (EPS) and enzymes in the digestion were analysed to understand the mechanisms responsible for this improvement. The results showed that the ultrasonic pretreatment enhanced enzymatic activities and accelerated the solubilisation of the EPS. In addition, ultrasonic pretreatment could not only break flocs and release extracellular materials but also destroy cells and release intercellular materials. With ultrasonic pretreatment, the sludge reduction for volatile suspend solids (VSS) in aerobic/anoxic digestion was 41.97±0.66%, compared with 22.81±1.40% for the control treatment, after an aerobic/anoxic digestion time of 10 d. Fourier transform infrared (FT-IR) spectroscopy results showed that the polymeric compounds, proteins and polysaccharides were easily hydrolysed and biodegraded in ultrasonic-treated sludge.