In this study, a novel biomaterial polyethyleneimine grafted H2O2‐oxidized starch nanocrystals (OSNCs‐PEI) is successfully prepared to remove hexavalent chromium (Cr(VI)) in the solution. The fourier transform infrared spectroscopy and energy dispersive X‐ray spectroscopy confirm that the hydroxyl groups on starch nanocrystals (SNCs) are oxidized to aldehyde groups, and PEI is successfully grafted on the OSNCs. When the pH is 3, the zeta potential of OSNCs‐0.1wt%PEI, OSNCs‐1.0wt%PEI, and OSNCs‐3.0wt%PEI reach the maximum value of +30.4, +43.2, and +46.3 mV, respectively. The effects of contact time, pH, initial concentration, and temperature on the adsorption behavior are investigated. The process of removing Cr(VI) from aqueous solution by OSNCs‐PEI is more suitable when pH is 3 and the adsorption time is 60 min. When the initial concentration of Cr(VI) is 1000 mg L–1, the adsorption capacity of Cr(VI) by OSNCs‐0.1wt%PEI, OSNCs‐1.0wt%PEI, and OSNCs‐3.0wt%PEI are up to 103.56, 173.91, and 184.35 mg g–1, respectively. The adsorption of Cr(VI) by OSNCs‐PEI follows the pseudo‐second‐order kinetic model and the adsorption experimental data fits well the Freundlich adsorption isotherm model. The adsorption of Cr(VI) by OSNCs‐PEI is a spontaneous and endothermic process.