The invention discloses a method for preparing conjugated linoleic acid from Idesia polycarpa Maxim. var. vestita Diels fruit oil. The problems that in the prior art, the yield for prepared conjugated linoleic acid is low, the impurity content is high, and residual solvents affect the human body health are solved. The method comprises the following steps that the Idesia polycarpa Maxim. var. vestita Diels fruit oil, a base catalyst and an alcohol-water solvent are added into a reaction container, and nitrogen is introduced to exchange air in the reaction container; a heating reaction is conducted under the stirring condition, and a reaction product system is obtained; the solvent is recycled by reducing pressure; acid is added to adjust the pH value to 3-5, standing is conducted for layering; the upper layer materials are taken to be washed with deionized water, and subjected to elution with 20%, 50%, 70% and 90% ethyl alcohol respectively, the 90% ethyl alcohol eluant is collected, solvent is vaporized thoroughly, and the conjugated linoleic acid is obtained. The process is simple, the operation is simple and convenient, the conjugated linoleic acid yield is high, the impurity content is low, and no ingredients harmful to the human body health are contained.