The invention discloses a forming device for a plastic ampoule. The device comprises a pressure head component and a forming component which are matched with each other, wherein the end of the pressure head component, which faces the forming component, is provided with a wedge surface which is matched with the forming component to form a twist-off groove on the inner wall of a bottle blank. The forming device has the advantages of simple and compact structure, low cost, high forming quality and the like, and is easy and convenient to operate.