A synthetic method for s-metolachlor. The invention relates to the technical field of synthesis process of the agricultural herbicide s-metolachlor. The method includes the steps of: 1) performing a mixing reaction to 2,6-methylethylaniline and methoxyacetone under the effects of a catalyst to obtain imine; 2) performing an asymmetry addition reaction to the imine and hydrogen under the effects of a chiral catalyst to obtain an imine hydride; and 3) performing an acylation reaction to the imine hydride and chloroacetyl chloride under the effects of an acid-binding agent, adding water to layer the reaction substances, and performing organic layer desolvation to obtain the s-metolachlor. The method has mild reaction conditions, high hydrogenation speed and simplified operation steps, is reduced in cost and is increased in yield. The product has high optical purity and can reach 90% in chiral content. The method can be used for prepare 97% s-metolachlor, wherein the total yield of the three steps reaches 90-92%.