The invention relates to an ageing oil demulsifier and a preparation method thereof, and belongs to the technical field of petroleum processing. The ageing oil demulsifier comprises mixed acid, a demulsifier monomer and an initiator, wherein the mixed acid consists of acrylic acid and maleic anhydride based on the molar ratio of (1-3): (0.5-1.5); the molar ratio of the mixed acid to the demulsifier monomer, namely, alkyl phenolic resin-polyoxyethylene polyoxypropylene ether, is (0.5-1.7): (0.5-1.5); the initiator, namely, lauroyl peroxide, accounts for 8 to 12% of the mass of the mixed acid. The preparation method comprises the following processes: adding the initiator, namely lauroyl peroxide to mixed acid and demulsifier monomer, namely alkyl phenolic resin-polyoxyethylene polyoxypropylene ether; polymerizing for 1 to 4 hours under a temperature of 70 to 100 DEG C; heating to reach 100 to 150 DEG C; esterifying for 3 to 7 hours to obtain the ageing oil demulsifier. The ageing oil demulsifier has the characteristics of being simple in components, simple in process, convenient, fast and efficient to prepare, low in cost, small in dosage, small in toxic and side effects, wide in application scope, good in demulsifying effect, etc.