The invention provides modified chitosan as well as a preparation method and application thereof, and relates to the technical fields of material preparation and separation. The modified chitosan is obtained by modifying chitosan by using 2-(hydroxymethyl)-12-crown-4; and the modified chitosan has a membrane structure. According to the modified chitosan provided by the invention, the chitosan is modified by using the 2-(hydroxymethyl)-12-crown-4, and the modified chitosan facilitates reducing the difficulty of extracting lithium ions while improving the selectivity to the lithium ions.