The invention discloses a method for removing sulfonic-group-containing dye in alkaline waste water by using a magnetic chitosan adsorbent, which comprises the following steps: (1) dissolving chitosan in an iron chloride water solution, stirring, adding ethanol to precipitate a solid, filtering out the solid, washing, mixing the solid with nano magnetic ferroferric oxide, carrying out cross-linking reaction with a glutaric dialdehyde water solution, and carrying out after-treatment to obtain a magnetic chitosan adsorbent; (2) adding the obtained magnetic chitosan adsorbent into an alkaline waste water solution containing the sulfonic-group-containing dye, stirring, carrying out magnetic separation, and drying to obtain the dye-adsorbed magnetic chitosan adsorbent; and (3) adding the dye-adsorbed magnetic chitosan adsorbent into a water solution containing a desorbing agent, stirring, and filtering to obtain a dye recovered solution and the recovered magnetic chitosan adsorbent. The method disclosed by the invention is simple to operate, has the advantages of environment friendliness and low cost, and has wide industrial application prospects.