The invention relates to a preparation method for a chitosan natural polymer modified crude oil desalting demulsifier. The method comprises the following steps: 1) modifying polyethylene glycol monomethyl ether by halogenating; 2) preparing glycidyl dimethyl alkyl ammonium chloride; 3) preparing quaternized carboxy alkyl chitosan; 4) causing the modified polyethylene glycol monomethyl ether to react with the quaternized carboxy alkyl chitosan, thereby obtaining a target product, polyethylene glycol monomethyl ether grafted quaternized carboxy alkyl chitosan. According to the invention, the chitosan natural high-molecular compound is taken as the raw material and has the advantages of wide resource, naturality, non-toxicity, sustainability, excellent biological compatibility of products, degradability, and the like. The demulsifier prepared according to the invention has excellent demulsification dewatering effect; the molecule of the demulsifier contains a large amount of carboxy alkyl groups and quaternary ammonium salt groups and the electronegative ions, including metal cation, naphthene acid radicals, and surfaces are electronegative grains and have strong combining capacity, so that the demulsifier has the capacities of demulsifying while removing oil-soluble salt.