The invention relates to a method for preparing a low sulfur diesel lubricity improver under a supercritical condition. Unsaturated carboxylic acid ester, vegetable oil fatty acids and a biocatalyst are added in proportion to a high-pressure reactor, carbon dioxide is introduced into the reactor to make the carbon dioxide in a supercritical state, stirring is started, and a reaction is carried outfor a certain period of time under a certain temperature and pressure; and after the reaction ends, the carbon dioxide is recovered and reused, filtration is performed on remaining liquid to remove the biocatalyst, and then an improver product is obtained by distillation under reduced pressure. The method utilizes the unsaturated carboxylic acid ester and the vegetable oil fatty acids as raw materials, and the improver product prepared by the method has a good lubrication effect, is low in product condensation point, and is low in blending ratio with low sulfur diesel, and can make the low sulfur diesel meet national V lubricity standards and condensation point requirements after blending.