The invention discloses a method for preparing an antioxidant 3,5-methyl ester. The method comprises the following steps: preparation of a catalyst, and preparation of the antioxidant 3,5-methyl ester. Methyl p-hydroxyphenylpropionate used as a raw material and tert-butanol undergo an alkylation reaction under a microwave condition under the action of the catalyst to obtain the antioxidant 3,5-methyl ester. Commonly-available and cheap clay is used as a raw material to prepare the novel catalyst, so waste of the catalyst is avoided, and the catalyst can be repeatedly used; and the methyl p-hydroxyphenylpropionate and tert-butanol are used as raw materials, so self-polymerization side reactions of the raw materials are avoided, and the prepared antioxidant 3,5-methyl ester has a high yieldand an excellent quality.