5-Chloromethylsalicylaldehyde was synthesized from salicylaldehyde, paraformaldehyde and concentrated hydrochloric acid as the raw materials and used with phosphorus oxychloride. The result showed that the yield of 5-chloromethylsalicylaldehyde reached 92% when the quality of phosphorus oxychloride is 30% of the salicylaldehyde, 15~25℃, 28h, stirred. Phosphorus oxychloride was not only used as the source of hydrogen chloride in the chloromethylation reaction, but also the regeneration for the mother liquor, which reduced the discharge value of waste acid. The raw materials cost to synthesize 5-chloromethylsalicylaldehyde reduced 25% compared to the references.