The present invention relates to the field of pharmaceutical chemistry, specifically to a preparation method of a weigh reducing drug lorcaserin hydrochloride and an intermediate thereof. The preparation method comprises: adopting p-chlorophenethylamine as a raw material, and sequentially carrying out acylation amino protection, allyl substitution, deprotection, Friedel-Crafts alkylation, splitting and salt forming to obtain the lorcaserin hydrochloride. According to the present invention, the preparation method has characteristics of low cost, simple operation and easy post-treatment, and is the economical and industrialized synthesis method.