An air delivery system is provided. The air delivery system includes a PAP device having a flow generator, a patient interface, a processor and a connector, and at least a removable sensing device having at least a sensor, a front-end analog circuit and a connector. The sensor is integrated with the patient interface to measure a respiratory characteristic in a path between the patients upper airway and the flow generator. When the removable sensing device is separated from the PAP device, the PAP device enters a first operation mode to perform a first preset air delivery behavior, and when the removable sensing device is coupled to the PAP device through the connectors respectively thereof, the PAP device enters a second operation mode to perform a second air delivery behavior for adjusting the treatment pressure in accordance with the respiratory characteristic.
