The invention relates to a method of preparing (2R)-2-propyl caprylic acid. The method comprises the following steps of: reacting (4R)-benzyl-1,3-oxazolidine-2-phenylimide with caprylyl chloride under the action of triethylamine and 4-dimethylamino-pyridine to obtain (4R)-benyl-N-capryloyl-1,3-oxazolidine-2-phenylimide; reacting the obtained compound with iodopropane to obtain (2'R, 4R)-benzyl-N-(2-propylcaprylyl)-1,3-oxazolidine-2-phenylimide; and releasing to obtain the (2R)-2-propyl caprylic acid. According to the method, chiral oxazolidine-2-phenylimide is used as a chiral auxiliary reagent to induce asymmetric alkylation reaction to synthesize the (2R)-2-propyl caprylic acid; the optical purity of the (2R)-2-propyl caprylic acid is greater than 98.7 percent e.e.; and the used chiral oxazolidine-2-phenylimide, serving as the chiral auxiliary reagent, can be recycled. The method has the advantages of feasible reaction line, simple post-treatment, capability of recycling the chiral auxiliary reagent, no need of catalytic reduction step and capability of avoiding recemization of a product.