The invention discloses a synthesis method of 2-chloro-5-chloromethylpyridine. The synthesis method comprises the following main steps: 1, using 3-methylpyridine (I) as a main starting material and water as a reaction medium, performing a complete reaction on the 3-methylpyridine (I) and chlorine gas at 40-60 DEG C in the presence of a catalyst, and post-treating to obtain 2-chloro-5-methylpyridine (II) as a reaction raw material in the next step; 2, using the 2-chloro-5-methylpyridine (II) as the reaction raw material, in the presence or absence of a reaction medium, performing a complete reaction on the 2-chloro-5-methylpyridine (II) and the chlorine gas at 50-60 DEG C in the presence of the catalyst, and post-treating to obtain 2-chloro-5-chloromethylpyridine with the purity being higher than or equal to 99%. A reaction equation is as shown in the description. The method disclosed by the invention has the advantages of high reaction selectivity, stable product quality, simple production operation, increase in raw material utilization rate through combined use of the raw materials, reduction in production cost, and the like.