The invention relates to a method for preparing a polymaleic anhydride fatty alcohol ether single ester leather fatliquoring agent. The preparation method comprises the following steps of: adding maleic anhydride and a peroxide initiator into a reactor; adding an organic solvent; introducing nitrogen (N2) to keep an anaerobic state; reacting to obtain maleic anhydride polymer; cooling; adding fatty alcohol ether and a catalyst; reacting and distilling under reduced pressure; neutralizing by using alkaline aqueous solution to obtain a product system; and adjusting solid content to obtain the polymaleic anhydride fatty alcohol ether single ester leather fatliquoring agent. The polymaleic anhydride fatty alcohol ether single ester leather fatliquoring agent prepared by the method is a surfactant with a Gemini type structure and has a good development prospect; the molecules of the polymaleic anhydride fatty alcohol ether single ester leather fatliquoring agent comprise a certain amount of ethyoxyl, so hydrophilcity is high and acid resistance and electrolyte resistance of the fatliquoring agent are improved; and the prepared product is polymaleic anhydride fatty alcohol ether single ester distributed in a certain molecular weight range and has a good application effect.