A synthesis method of metolachlor comprises the steps of: (1) hydrogenation alkylation: dissolving2-methyl-6-ethyl aniline and methoxy acetone with an organic solvent A, adding a hydrogenation catalyst, introducing hydrogen to conduct a hydrogenation alkylation reaction, and concentrating the solvent after the reaction to obtain a reaction product B, wherein the hydrogenation catalyst is a composition of palladium carbon, Raney nickel and phosphoric acid in the weight ratio of 1-2:4-10:2-5, and the temperature of the hydrogenation alkylation reaction is 30-90 DEG C; and (2) acylation: conducting an acylation reaction on the product B with chloroacetyl chloride under the catalysis of an acid-binding agent, and conducting post-treatment after the reaction to obtain refined metolachlor. The synthesis method of metolachlor provided by the present invention has the advantages of simple operation, short production cycle, small amount of organic reagent and emission, high yield and purity of the desired products, no additional solvent in the post-treatment process, reduction of "three wastes" emissions; and the production process is safe, environmentally friendly and applicable to mass industrial production.