The invention discloses a method for discoloring mineral insulating oil by using an alkaline chitosan film. The method comprises the following steps: 1) dispersing chitosan powder into a solvent system obtained by mixing alkali and urea, sequentially freezing, unfreezing and defoaming to obtain a chitosan alkaline solution; adding epoxy chloropropane into the chitosan alkaline solution for cross-linking and defoaming so as to obtain a chitosan pre-gel solution; casting the chitosan pre-gel solution into a mold for performing film molding, and finally washing and drying the molded film so as toobtain an alkaline chitosan film; 2) pouring to-be-treated mineral oil into a reactor, adding the alkaline chitosan film obtained in the step 1) to be mixed, raising the temperature to 60-80 DEG C, reacting for 8-12 hours, cooling and filtering, thereby obtaining the discolored mineral oil. The alkaline chitosan film prepared in the invention has high desorption property, and the method is simplein discoloring step, convenient to operate, green and environmentally friendly and is suitable for large-scale application.