A method is provided for forming a flash memory. The method includes providing a semiconductor substrate; and forming a first dielectric layer. The method also includes forming a first semiconductor layer on a surface of the first dielectric layer; and performing an ion implantation onto a portion of the first semiconductor layer corresponding to a position of a subsequently formed floating gate. Further, the method includes performing an oxygen ion implantation process onto a portion of the first semiconductor layer between the position of the subsequently formed floating gate and the position of a subsequently formed first select gate to form an oxide layer; and forming a second dielectric layer having an opening exposing the position of the first select gate. Further, the method also includes forming a second semiconductor layer on the second dielectric layer; and forming a flash cell and a select gate structure.