Chitosan is a polymer obtained by deacetylation of the natural polysaccharide chitin. It has various physiological functions such as biodegradability, biocompatibility, and non-toxicity. The hydroxyl and amino groups in their molecules can form active interfaces to adsorb dye molecules. However, direct application is limited due to its weak mechanical strength and chemical stability, thus requiring chemical modification. In this work, chitosan hydrogel beads (CCS-DETA) were prepared by sol-gel method using chitosan (CS) as matrix, glutaraldehyde (GA) as crosslinking agent, and diethylenetriamine (DETA) as modifier. The adsorption performance of CCS-DETA hydrogel beads on methyl orange (MO) dye in wastewater was studied. Effects of pH value, initial concentration, adsorption isotherm, and kinetics were discussed with mathematical model fitting results. The microstructure and adsorption mechanism of CCS-DETA hydrogel beads were characterized by Fourier transform infrared spectroscopy (FTIR), field emission scanning electron microscope (FESEM), and X-ray photoelectron spectroscopy (XPS). The results showed that CCS-DETA was protonated under low pH values that electrostatically interacted with negatively charged MO molecules in the adsorption process. It was showed that CCS-DETA was protonated at low pH and electrostatically interacted with negatively charged MO molecules during the adsorption process. Furthermore, at higher pH, O and N ions on MO can form hydrogen bonds with hydroxyl groups on CCS-DETA. The adsorption results showed that CCS-DETA hydrogel beads had excellent adsorption effect on MO. When the dosage of CCS-DETA hydrogel beads was 30 mg, the initial concentration of MO was 20 mg/L, pH was 2, and the reaction time was 12 h, the MO removal rate could reach 91.33%, and CCS-DETA hydrogel beads had exhibited good stability. The adsorption kinetics and thermodynamics showed that the adsorption process was more consistent with the pseudo-first-order kinetics model and Langmuir isothermal adsorption model. The maximum adsorption capacity of MO reached 209.68 mg/g at 313.15 K. This study suggested that CCS-DETA can efficiently adsorb MO from aqueous solutions via amino and hydroxyl groups.