Disclosed is a process for the synthesis of pimavanserin base with a high yield and purity, which comprises: a) converting tert-butyl-N-[(4-propan-2-yloxyphenyl)methyl]carbamate (Formula (I)) to 1-(isocyanatomethyl)-4-propan-2-yloxybenzene of formula (II) b) adding N-[(4-fluorophenyl)methyl]-1-methylpiperidin-4-amine (Formula (IV)) to the solution obtained in a) to give pimavanserin base, and c) purifying the pimavanserin base obtained in step b).