Provided is an expansion electronic device suitable for being disposed at an edge of a main framework having therein a motherboard. The expansion electronic device includes an expansion frame bar and functional module. The expansion frame bar has thereon an expansion box. The expansion box has an access opening. The inner rim of the access opening has an inner annular wall. The access opening is covered with a protective cover. The outer rim of the protective cover has an outer annular wall corresponding in shape to the inner annular wall. A resilient annular surface is defined on the outer surface of the outer annular wall. The resilient annular surface abuts against the inner annular wall to hermetically seal the access opening. The functional module is disposed in the expansion box and electrically connected to a control wire exiting the functional module and electrically connected to the motherboard.