The invention discloses a method for ultrasonic assisted conversion of chitin into chitosan. The method comprises the following steps: (1) adding deionized water into lucid ganoderma spore powder, carrying out ultrasonic extraction in a water bath, then adding ethanol, carrying out ultrasonic extraction at a room temperature, carrying out centrifuging, then carrying out soaking with deionized water, and carrying out centrifuging and drying so as to obtain crude chitin; (2) placing the crude chitin in a H2O2 aqueous solution, maintaining the water bath until decolorizing of the crude chitin iscompleted, adjusting a pH value of a decolorized chitin solution to neutral with a NaOH solution, and carrying out centrifuging so as to obtain a precipitate; and (3) adding an alkali solution into the precipitate, subjecting a mixed solution to ultrasonic treatment so as to complete a deacetylation reaction, adjusting a pH value of an obtained solution to neutral with a HCl solution, then carrying out centrifuging so as to obtain a precipitate, and carrying out drying so as to obtain a crude chitosan product. The method provided by the invention uses NaOH with a small amount and a low concentration and combines an ultrasonic process to prepare the chitosan at a room temperature; in addition, the method fully utilizes residue by-products, broadens the source of the chitosan, and is applicable to scale production of the chitosan.