The invention relates to a preparation method of lactic ester. The method includes: mixing a carbohydrate, a transition metal salt, alcohol and water, subjecting the mixed solution to stirring reaction under a temperature of 200-260DEG C and a pressure of 4-15MPa so as to obtain the lactic ester. The transition metal salt is acidic, the alcohol is low carbon alcohol with a carbon atom number of 1-4, and the carbohydrate has a hydrolysis product containing monosaccharide. The preparation method of the lactic ester provided by the invention has the advantages of simple overall reaction process, safety and high efficiency, low cost, low equipment requirement, popularization and application, and significant increase of the product conversion rate.