A mobile electronic device includes a main framework having a motherboard, a back cover covering the main framework and having an expansion window, and an expansion unit is disclosed. The expansion unit includes a door cover covering the expansion window. The door cover includes a back plate and a lateral plate, the lateral plate having an opening. A functional module is disposed on an inner side of the door cover, electrically connected to the motherboard, and has a first side on which an operation interface is disposed. The operation interface faces the opening in the lateral plate.