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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 C N to –NH– owned the best performance. C N 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 C N accelerated itself hydrolysis and leaded to the decomposition of CTS-g-PAM, deteriorating the performance of the flocculant. Image 1 • Two different structures of CTS-g-PAM were discovered via XPS. • The ratio of two structures was different in various synthesis conditions. • The moderate C N in CTS-g-PAM contributed to its better performance in the treatment. • C N reinforced the interaction between CTS-g-PAM and azo dyes. • Excessive C N accelerated itself hydrolysis and decomposition of CTS-g-PAM. [ABSTRACT FROM AUTHOR]