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 for supplying a breathable pressured air to a patient and a sensing device to acquire physiological information from the patient by at least a sensor. The PAP device has two operations modes which are exchangeable while the air is supplied to the patient based on the connection or disconnection of the sensing device. When the sensing device is not connected to the PAP device, the PAP device enters a preset independent operation mode and performs a preloaded first air delivery behavior. When the sensing device is not connected, the PAP device enters a common operation mode, and during the common operation mode, based on the acquired physiological information, the sensing device generates a signal/data for deciding a second air delivery behavior of the PAP device.
