A wearable device including a skin sensor and a processor is provided. The processor is configured to receive an authentication data for authenticating a user when a wearing state of the wearable device is adjacent to a skin surface of the user, execute a predetermined function in response to a request when the authentication data matches a pre-stored data and the skin sensor determines that the wearable device does not leave the skin surface after the authentication data is received, and reject or ignore the request when the skin sensor determines that the wearable device leaves the skin surface before the predetermined function is executed. The processor further calculates blood pressures according to PPG signals detected by a PPG sensor of the skin sensor.