In this paper the Mannich reaction product of dimethylamine, acetone and formaldehyde was grafted onto hydroxymethylated lignin to form a lignin-base cationic polyelectrolyte (L-DAF). L-DAF samples with different cationic strengths and molecular weights were synthesized and characterized. Their performance as a cationic flocculant in removing three anionic azo-dyes (Acid Black, Reactive Red and Direct Red) from simulated dye wastewater was evaluated. The removal of anionic dyes by L-DAF was ascribed to two simultaneous mechanisms, which included coagulation by charge neutralization and flocculation by bridging. There was a different optimum dose of flocculant for each dye wastewater, at which dye removal can reach above 95%. Furthermore, a longer settling time and a lower solution pH increased the efficiency of the decolorization process. During the treatment, more than 89% of chemical oxygen demand (COD) reduction was achieved with sludge production being less than 5.4%.