An electronic device and method for operating system waking up thereof are provided. The electronic device includes a processing unit, a timer coupled to the processing unit and a basic input/output system unit (BIOS) coupled to the processing unit. When an operating system operated by the processing unit is in a sleep state and the processing unit receives a wake-up signal, the processing unit executes a wake-up procedure through the BIOS to switch the operating system from the sleep state to a work state, and also controls the timer to start a timing procedure. When the processing unit determines that a timing value of the timing procedure exceeds a threshold and the operating system fails to switch from the sleep state to the work state, the processing unit executes a reset procedure through the BIOS.