In this work, four alkaline lignin (AL) based flocculants with distinct molecular weight, chain architectures and charge densities (denoted as AL-g-DMC1, AL-g-DMC2, AL-GTA1 and AL-GTA2) were prepared from paper mill sludge, which were designed via graft copolymerization of dimethyl diallyl ammonium chloride (DMC) or etherification of 2, 3-epoxypropyl trimethyl ammonium chloride (GTA). The characteristics of the aforementioned flocculants were evaluated by a series of analysis technologies, which essentially confirmed the successful introduction of quaternary ammonium groups onto the AL. The flocculation performances of the four synthesized lignin-based polymers as the coagulant aids for PAC were investigated in disperse dye (DY) wastewater treatment, and the effects of dosages, initial pH, coexisting ions, humic acid (HA) or kaolin particles were also studied. The results indicated that branched copolymers with high molecular weight like AL-g-DMC1 and AL-g-DMC2 exerted excellent color removals and satisfactory floc properties in comparison with linear polymers with low molecular weight (AL-GTA1 and AL-GTA2). Furthermore, AL-g-DMC1 and AL-g-DMC2 exhibited remarkable tolerance on pH alteration and coexisting ions owing to their strong bridging action. Display omitted •Four lignin-based flocculants were prepared from paper mill sludge.•Flocculants with distinct molecular weight and charge densities were designed.•Branched copolymers with high molecular weight exerted excellent color removals.•Flocculants with same chain architectures but higher charge density performed well.•The synergetic mechanisms and structure-activity relationships were discussed.