An electrical connector assembly of the invention comprises a butt plug with a contact portion and an electrical connector capable of matching the butt plug. One side of the contact portion has at least one engaging point. The electrical connector comprises an insulating body and a shield casing with at least one bottom wall. A side wall extends upward from at least one side of the bottom wall and the side wall covers at least one part of the insulating body. An opening, which is located at the front of the bottom wall and the side wall, is used for receiving the contact portion. The bottom wall has at least one fastening hole matching the engaging point. Enable the engaging point to slide into the corresponding fastening hole in parallel. When the butt plug is inserted into the shield casing once, the invention can ensure good electrical connection.