The invention discloses a preparation method of pimavanserin. The method comprises the following two steps: firstly, 4-isobutoxy benzene methylamine and carbonyl diimidazole are subjected to acylation reaction to obtain N-(4-isobutoxy phenyl)-1H-imidazole-1-formamide, and the N-(4-isobutoxy phenyl)-1H-imidazole-1-formamide and N-(4-fluorophenyl)-1-methylpiperidine-4-amine are subjected to urea reaction, so as to obtain the pimavanserin. The prepared pimavanserin is good in quality and high in yield, the reagent toxicity is relatively low, the operation is simple and easy to control, and the pimavanserin is suitable for industrial production.