Abstract: In order to imporve the activated sludge settling performance by using chironomid larvae(CL),the effect and mechanism of CL on activated sludge settling performance were investigated in this study.The results show that the adding of CL enhances the sludge settling performance in a short time.The improvement performance becomes significant as the CL density increased from 0.2 to 1.2 g wet mass of CL per g dry mass of activated sludge.There is a suitable sludge concentration(5 g/L) in achieving settleability improvement at the same CL density(1.0 g wet mass of CL per g dry mass of activated sludge).The introduction of CL reduces the contents of protein and polysaccharide and the ratio of protein to polysaccharide in extracellular polymeric substance(EPS),which is probably the main reason for CL improving the sludge settling performance.Furthermore,the long-term experiment confirms that adding the supernatant from the CL reactor to the sequencing batch reactor(SBR) for treating domestic wastewater reduces the sludge volume index(SVI) from 120 mL/g to 60 mL/g.