To enhance the adsorption performance, chitosan microspheres (CS-MSs) were grafted with polyethylenimine (PEI), a cationic compound with abundant amine groups. The resulting PEI-grafted CS-MSs (PEI-CS-MSs) were used for the adsorption of xylenol orange (XO), an anionic dye. The results show that PEI-CS-MSs were in regular spherical shape without porous structure. XO molecules were adsorbed onto the amine groups of PEI-CS-MSs through electrostatic interaction. The maximum adsorption capacity of XO onto PEI-CS-MSs was 816.33mg/g at 303K, being much higher than that onto CS-MSs. The adsorption process could be properly described by the pseudo-second-order kinetic and Langmuir models. The adsorption of XO was a spontaneous process with exothermic nature. In addition, the adsorption capacity only decreased by 0.52% after seven cycles of adsorption-desorption, indicating the excellent reusability of PEI-CS-MSs.