A device of drowsiness detection and alarm includes a headrest, at least one brainwave sensor, a computing unit, a controlling unit and an accessory unit. The brainwave sensor is disposed on the headrest and configured to contact with a user's head and detect a brainwave signal. The computing unit receives the brainwave signal from the brainwave sensor and calculates according to the brainwave signal to obtain a fatigue index. The controlling unit controls the accessory unit to activate according to the fatigue index. Therefore, when the user is drowsy and gradually loses consciousness, the accessory unit can immediately alarm the user and allow the user stay awake to avoid the occurrence of accidents. A method of drowsiness detection and alarm is also disclosed.
