Excess biological sludge was utilized to prepared bioflocculant with hydrochloric acid. The prepared crude bioflocculant was purified and fractionally precipitated to attain four purified sludge bioflocculant defined as PSB1-4. The PSB-2 has higher flocculating rate for kaolin suspension than others. When the pH of the flocculation system ranged from 4.0 to 11.0 the flocculating rates of PSB-2 were over 96.0%. X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FTIR) spectra showed that amino and hydroxyl groups were present in the bioflocculant molecules. More amine group existed in the bioflocculant PSB-2 relatively. The amino group was believed to play an important role in flocculation. The experiment of zeta potential measuring indicated that the charge neutralization contributed to flocculation process. Flocculating mechanism investigation reveals that the sludge bioflocculant caused kaolin suspension instability by means of charge neutralization firstly and then promoted the aggregation of suspension particles by adsorption and bridge. Crown Copyright (c) 2013 Published by Elsevier Ltd. All rights reserved.