Agricultural byproducts have excellent potential for pollutant remediation due to their low-cost and environmental sustainability. Konjac flying powder (KFP) is a fine particulate byproduct of the production of konjac glucomannan, accounting for 30–40 wt% of the total raw materials. In this work, KFP was modified by free radical grafting copolymerization with acrylic acid, methacrylic acid and N,N′-methylene bis(acrylamide). The lightly crosslinked product demonstrated excellent adsorption performance for heavy metals, with the maximum adsorption capacity for Cd(II), Pb(II) and Cu(II) reaching 635 mg g⁻¹, 812 mg g⁻¹, and 855 mg g⁻¹, respectively, under optimized conditions. The adsorption isotherm of heavy metal ions fitted well with the Langmuir model. Moreover, the adsorption performance was nearly unchanged after several adsorption–desorption cycles, suggesting that polymeric derivation of KFP has great potential for the remediation of heavy metal pollution.