The invention discloses a preparation method of an amorphous O-doped BN nanosheet. By adopting a hard template method and taking an amorphous graphene like CuB23 nanosheet as a template agent, 2CuB23+46NOC1 arrow 46BNO+2CuC12+21C12 is successfully synthesized into the O-doped BN nanosheet at a room temperature through one-step reaction and is taken as an electrode for electrical adsorption of heavy metal ions in water. The amorphous O-doped BN nanosheet has a unique structure and coordination ability, can completely and quickly remove the various heavy metal ions in the water simultaneously and particularly has an excellent ability to remove Cd<2+> and Zn<2+>. The amorphous O-doped BN nanosheet has wide application prospect in the fields of water purification and water softening based on a CDI technology.