Chitosan-based flocculants are efficient and biodegradable, possessing the potential application in water treatment. A chitosan-based flocculant, CTS-g-PAM, was prepared via grafting copolymerization. Two main structures of CTS-g-PAM were detected by X-ray photoelectron spectroscopy (XPS), and they related to the different performances of CTS-g-PAM in the treatment of model azo dyeing wastewater in terms of removal efficiency, floc size and zeta potential. CTS-g-PAM with the optimal ratio of Cdouble bondN to –NH– owned the best performance. Cdouble bondN reinforced the interaction between CTS-g-PAM and azo dyes and charge neutralization while the flocs formed in the case were large and compacted. However, excessive Cdouble bondN accelerated itself hydrolysis and leaded to the decomposition of CTS-g-PAM, deteriorating the performance of the flocculant.