In current work, lignin-based multi-arm star copolymer phenolated kraft lignin-graft-poly (2-methacryloyloxyethyl) trimethyl ammonium chloride (PKL-g-PDMC) was designed and synthesized as a new kind of cationic flocculant via Steglich esterification of PKL and subsequent RAFT polymerization with DMC, and its flocculation performance in the removal of kaolin particles from simulated wastewater was evaluated. The star PKL-g-PDMCs with varying graft density and arm length of PDMCs chains were characterized by H-1 NMR, P-31 NMR, GPC and TGA. The effects of graft density and arm length, PKL-g-PDMC dosage, suspension pH and settling time on flocculating performance were investigated by measuring residual turbidity of the settled suspension. The treatment was effective, as demonstrated by 96.4% efficiency of turbidity removal. Above all, the chemical structure of the copolymers, including graft density and the arm length, can be designable and tunable by simply changing feeding ratios of Steglich esterification and subsequent RAFT polymerization, and thus the best flocculation performance can be obtained. This work provides a blueprint for the design, synthesis and use of novel lignin-based multi-arm star copolymers for wastewater treatment with tunable properties.