The invention relates to a crude oil demulsifier, and discloses a natural polysaccharide macromolecule-modified crude oil demulsifier. The demulsifier is prepared by adopting the following operation steps: (1) conducting halogenation modification, carboxylation modification, amination modification, formylation modification, or isocyanic acid esterification modification on polyethylene glycol monomethyl ether; (2) preparing glycidyl dimethyl alkyl ammonium chloride; (3) preparing quaternized polysaccharide; and (4) reacting the modified polyethylene glycol monomethyl ether with quaternized polysaccharide to obtain a product, namely the natural polysaccharide macromolecule-modified crude oil demulsifier. According to the crude oil demulsifier, as the raw material, the natural polysaccharide macromolecule compound has multiple advantages of being wide in source, natural, non-toxic, sustainable, good in usage safety and the like; and the prepared demulsifier has good demulsifying and dewatering effects; in addition, the molecules of the demulsifier contain a great deal of groups of hydroxyl and the like, which have certain chelating capability to metal ions, thus being capable of removing a certain amount of metal ions while demulsifying.