The invention discloses a preparation method of a diesel oil pour point depressant. The method includes the following steps of firstly, weighing high alcohols, catalyst, polymerization inhibitor and solvent to be sequentially added into a reactor and stirred till solids are completely dissolved, then adding methacrylic acid, and conducting reduced pressure distillation, alkali washing and water washing on the product after reaction ends to obtain methylacrylic acid high-carbon ester; secondly, weighing methylacrylic acid high-carbon ester, maleic anhydride, initiator and solvent to be evenly mixed, conducting reacting at a certain reaction temperature under the protection of nitrogen, adding aromatic amine and catalyst after reaction, changing the reaction temperature for reaction for a certain period of time, conducting reduced pressure distillation on the reaction liquid after reaction ends, then adding an excessive amount of methyl alcohol to generate white precipitates, removing supernate, and conducting washing and vacuum drying on the obtained white precipitates through methyl alcohol to obtain the diesel oil pour point depressant. The prepared diesel oil pour point depressant can remarkably reduce the condensation point of diesel oil and improve the low-temperature flowability of diesel oil; the raw materials are easy to obtain, the synthesis method is simple, and the additive dosage is small.