An extendable air delivery system and a method for deciding a delivery mode therein are provided. The extendable air delivery system includes a PAP device supply a breathable pressured air to a patient and a sensing device to acquire physiological information from the patient by at least a sensor. The method includes steps of initiating the PAP device, detecting if the sensing device is connected with the PAP device, and if negative, the PAP device enters a preset independent operation mode and performs a preloaded first air delivery behavior for providing the breathable air to the patient, or if positive, the PAP device enters a common operation mode. During the common operation mode, based on the acquired physiological information, the sensing device generates a signal/data for sending to the PAP device, and the signal/data involves in deciding a second air delivery behavior for providing the patient the breathable air. And, the above steps are repeated during the air delivery.
