An extendable air delivery system, including a PAP device and a sensing device, wherein: the PAP device comprises: a flow generator, for providing a breathable air to a patient; a processor, for providing a control signal to the flow generator; and a digital communication module, controlled by the processor; and the sensing device comprises: at least a sensor, for acquiring a physiological information from the patient; a processor, for obtaining the physiological information from the sensor and generating a signal/data according thereto; and a digital communication module, controlled by the processor, the PAP device is configured to be one selected from a CPAP device, a BiPAP device, an APAP device and a VPAP device, and the PAP device and the sensing device are configured to be connected to or separated from each other; and wherein when the sensing device is not connected with the PAP device, the PAP device enters a preset independent operation mode and performs a preloaded first air delivery behavior of CPAP device, or BiPAP device, or APAP device, or VPAP device; and when the sensing device is connected with the PAP device, the PAP device enters a common operation mode, and during the common operation mode, the PAP device and the sensing device communicate via the corresponding digital communication modules thereof, for sending the signal/data from the sensing device to the PAP device, thereby deciding a second air delivery behavior of the flow generator, wherein the second air delivery behavior is an air delivery behavior of BiPAP device, or APAP device, or VPAP device; and the PAP device is configured to check the connection with the sensing device to decide the operation mode. the PAP device comprises: a flow generator, for providing a breathable air to a patient; a processor, for providing a control signal to the flow generator; and a digital communication module, controlled by the processor; and a flow generator, for providing a breathable air to a patient; a processor, for providing a control signal to the flow generator; and a digital communication module, controlled by the processor; and the sensing device comprises: at least a sensor, for acquiring a physiological information from the patient; a processor, for obtaining the physiological information from the sensor and generating a signal/data according thereto; and a digital communication module, controlled by the processor, at least a sensor, for acquiring a physiological information from the patient; a processor, for obtaining the physiological information from the sensor and generating a signal/data according thereto; and a digital communication module, controlled by the processor, the PAP device is configured to be one selected from a CPAP device, a BiPAP device, an APAP device and a VPAP device, and the PAP device and the sensing device are configured to be connected to or separated from each other; and wherein when the sensing device is not connected with the PAP device, the PAP device enters a preset independent operation mode and performs a preloaded first air delivery behavior of CPAP device, or BiPAP device, or APAP device, or VPAP device; and when the sensing device is connected with the PAP device, the PAP device enters a common operation mode, and during the common operation mode, the PAP device and the sensing device communicate via the corresponding digital communication modules thereof, for sending the signal/data from the sensing device to the PAP device, thereby deciding a second air delivery behavior of the flow generator, wherein the second air delivery behavior is an air delivery behavior of BiPAP device, or APAP device, or VPAP device; and the PAP device is configured to check the connection with the sensing device to decide the operation mode.