A flexible display device includes: first and second casings; a frame movably assembled to the second casing; and a flexible display having first and second ends opposite to each other and first and second display areas. The second casing is movably assembled to the first casing to form a drawer configuration and slide relative to the first casing to switch between drawn-out and retracted states. The first end is connected to the first casing. The second end to the frame. In the retracted state, the flexible display is bent so the second display area is contained in the first and second casings. In a process from the retracted state to the drawn-out state, the second casing is drawn out from the first casing and drives the frame to move in the same direction to expand and flatten the second display area. A portable electronic device is also provided.