The invention provides a preparation method of a magnetic composite hydrogel based on cellulose nanofiber. According to the invention, a natural high-molecular material cellulose acetate having good biological compatibility is used as a raw material, N-isopropylacrylamide having carboxyl and amino groups and acrylic acid are grafted onto cellulose acetate nanofiber to prepare the cellulose nanofiber grafted poly(N-isopropylacrylamide-co-acrylic acid) hydrogel, and finally, the cellulose nanofiber grafted poly(N-isopropylacrylamide-co-acrylic acid)/Fe3O4 magnetic composite hydrogel is obtainedby a coprecipitation method. The raw materials are recyclable and easy to obtain, and the preparation method is simple, and is suitable for industrial production. The synthesized hydrogel has good biocompatibility, low price and strong adsorption of heavy metal ions in sewage.