Aminated lignin (AL) was synthesized by a Mannich reaction between hexane-diamine (HD) and lignin with the assistance of ultrasound. The structure and properties of AL were characterized by Fourier transform-infrared spectroscopy (FT-IR) and thermogravimetric analysis (TGA). The effect of AL content, initial dye concentration, pH and settling time on the dye decolorization, and chemical oxygen demand (COD) removal were studied. The decolorization efficiency and flocculation capacity of AL were obviously better than that of lignin. Coagulation–flocculation models of AL were studied. The adsorption isotherms of AL for Congo red and EBBR in aqueous solutions could be fitted by the Langmuir model, and the adsorption kinetics were closer to a pseudo second-order equation. Its performance as a cationic flocculant in removing two anionic azo-dyes (Congo red and Eriochrome blue black R (EBBR)) from simulated wastewater was studied. There was a different optimal dose of flocculant for each dye in wastewater, at which dye removal could reach 96%.