A portable electronic device and a control method thereof are provided. The method is adapted to a portable electronic device including a rotatable element, a fixed element, a screen, a processor, a switch module with at least two protruding elements, and fixed points. The rotatable element is nearby the fixed element. The protruding elements and the fixed points are facing to each other and configured at one and the other of the rotatable element and the fixed element. The processor is coupled to the switch module and the screen. The method includes to display a default user interface corresponding to a default mode on the screen, to determine a corresponding mode according to all press states of each protruding element in the switch module by the processor when the rotatable element is rotated due to an external force applied thereto, and to display a corresponding user interface on the screen.