In order to explore the strengthening effect of low-intensity ultrasound on the anaerobic biological treatment of low-strength wastewater, a research on the improvement of organic matter removal by ultrasonic irradiation of anaerobic sludge in anaerobic baffled reactor (ABR) was carried out. The effects of ultrasound on sludge yield, extracellular polymeric substances (EPS), enzyme activity, sludge particle size, sludge surface functional groups and microscopic morphology were further studied.The results showed that low-intensity ultrasound could improve the removal of organic matter in low-strength wastewater by ABR. The removal efficiency of chemical oxygen demand (COD) in the ultrasound group was 5.2% higher than that of the control group. The COD concentration of the effluent could meet the first-class A standard in the Pollutant Discharge Standard for Urban Sewage Treatment Plants (GB18918-2002). After periodic ultrasound, the total suspended solids (TSS) and volatile suspended solids (VSS) of each compartment of the ultrasound group were lower than those of the control group, but the VSS/TSS ratio was higher. The EPS increased in the ultrasound group, while the loosely bound EPS (LB-EPS) and tightly bound EPS (TB-EPS) increased. The protein (PN) content increased but the polysaccharide (PS) content decreased. The dehydrogenase activity (DHA) of each compartment of the ultrasound group was 26.43mgTF/(gVSS·h), 23.43mgTF/(gVSS·h), 21.87mgTF/(gVSS·h), 19.55mgTF/(gVSS·h), while the control group was 18.13mgTF/(gVSS·h), 17.01mgTF/(gVSS·h), 13.56mgTF/(gVSS·h), 9.90mgTF/(gVSS·h). Ultrasound greatly improved the activity of anaerobic sludge dehydrogenase. Ultrasonic treatment reduced the particle size of the sludge, but there was basically no change in the types of surface functional groups. Scanning electron microscope (SEM) observation showed that filamentous bacteria were the dominant strain on the sludge surface of each compartment in the control group, while cocci were the dominant strain on the sludge surface of each compartment in the ultrasound group. © 2021, Chemical Industry Press Co., Ltd. All right reserved.