The invention discloses a method for preparing a leather retanning fat-liquoring agent which is a long chain aliphatic alcohol n-butyl alcohol maleic diester methacrylate polymer prepared by a water emulsion copolymerized reaction of a long chain aliphatic alcohol n-butyl alcohol maleic diester and a methacrylic acid. The leather retanning fat-liquoring agent of the invention is a high molecule material with an amphipathic structure, is suitable for the leather processing, has multifunction of retanning, fat-liquoring, water proof and wash resistance or the like, reduces the leather chemical material amount and the tanning process and is useful for implementing leather cleaning production.