The invention discloses a preparation method of lorcaserin intermediate I, which uses p-chlorophenylacetonitrile as a starting material to prepare lorcaserin intermediate I through reduction and condensation steps. The starting materials of the present invention are p-chlorophenylacetonitrile and 1-chloro-2-propanol, which are cheap and easy to obtain, avoid the use of thionyl chloride, hydrobromic acid, borane and other easily polluting and explosive raw materials, and avoid producing a large amount of industrial waste water , is more conducive to environmental protection, reduces the labor protection requirements of workers, and ensures safe production. The route is novel in design, easy to obtain raw materials, simple and feasible in process operation, and environment-friendly, and provides a simple and feasible process method for large-scale industrial production.