An (acid-substituted polyaniline)-grafted hydrogel copolymer is provided. The (acid-substituted polyaniline)-grafted hydrogel copolymer has a general formula as below: wherein A is a proton acid group, m and n are same or different integers greater than 0, x is an integer equal to or greater than 0, and y is an integer equal to or greater than 1, provided that x and y are the same or different at each occurrence, and at least an x is not 0. The (acid-substituted polyaniline)-grafted hydrogel copolymer is formed by the polymerization and substitution reaction between chitosan, polyaniline, and proton acid. The (acid-substituted polyaniline)-grafted hydrogel copolymer behaves as a pH-responsive hydrogel with photo-thermal properties and can be applied to photo-thermal therapy.