The invention discloses a method for preparing EPDM through a rare earth catalytic system, and belongs to the technical field of functional polymer materials. The preparation method comprises the following steps that a solvent is added into a reaction kettle with no water or oxygen, 160-400 microlitre of aluminum alkyl, stirring is performed for 5-10 min, ethylene/propylene mixed gas is introduced according to the ratio, a third monomer is added, the reaction is performed under normal pressure or micro-positive pressure, then, 20-80 mg of a non-cyclopentadienyl rare earth alkyl compound is added, stirring is performed for 2-15 min, 22-85 mg of an organic borate reagent is added, a polymerization reaction is performed under the temperature of 10-60 DEG C for 30-360 min, after polymerization is finished, a terminating agent and an anti-aging agent are added, a large amount of ethyl alcohol is used for cleaning, and the mixture is placed into a vacuum drying box to be dried to obtain EPDM. According to the method, the catalytic system composed of the non-cyclopentadienyl rare earth alkyl compound, aluminum alkyl and the organic boron reagent is adopted, and a solution polymerization reaction is performed in the solvent to obtain the EPDM. According to the method, the aluminum alkyl reagent is introduced, the molecular weight of the product is controllable, the catalytic efficiency is improved, and the obtained rubber molecular weight distribution is narrow.