A mobile electronic device includes a main framework, a back cover, a door cover, a functional module and a stop element. The back cover covers the main framework and has an expansion window. The door cover covers the expansion window. The functional module is disposed on an inner side of the door cover and electrically connected to a motherboard in the main framework. An operation interface corresponding in position to an outer side of the door cover is disposed on one side of the functional module. One side and the other side of the functional module oppose each other. The stop element is disposed on the inner side of the door cover, abuts against the other side of the functional module, and thus prevents the functional module from retracting under an external force. Therefore, the mobile electronic device is insusceptible to a collision and a fall.