A handheld electronic device is provided. The handheld electronic device includes a touch display unit, a sensor unit and a processing unit. The sensor unit detects a rotation angle of the handheld electronic device relative to a gravity direction and a displacement of the handheld electronic device. The processing unit is electrically connected to the touch display unit and the sensor unit to extend a timeout threshold of the touch display unit from a first value to a second value that is larger than the first value when the rotation angle is within a reading mode range. The processing unit keeps the timeout threshold at the second value when at least a first condition that the displacement of the handheld electronic device is larger than a predetermined value is met.