Belonging to the field of pharmaceutical chemistry, the invention discloses a synthesis method of lorcaserin. The method includes: taking 4-chlorophenethylamine as the raw material, carrying out reductive amination reaction with acrolein to prepare N-allyl-4-chloro-phenethylamine (compound 2), protecting the amino of compound2 by with t-butyloxycarboryl (Boc) to obtain N-Boc-N-allyl-4-chloro-phenethylamine (compound 3), subjecting the compound 3 to palladium acetate catalyzed cyclization to obtain a compound 4, conducting catalytic hydrogenation on the compound 4 to obtain N-Boc-lorcaserin, removing the Boc group under an acidic condition to obtain a lorcaserin crude product, and performing further purification to obtain lorcaserin. The synthesis method provided by the invention has the advantages of cheap and easily available reagent, safe and reliable reaction, recyclable reaction solvent, simple post-treatment operation, and environment-friendliness, and is conducive to large-scale industrial production.