The invention discloses an expiration NO detector, comprising three modules: a sampling module, a detection module and a control module; the sampling module complies with programs recommended by international expiration detection standard, automatically controls the aspiration quality, flux and pressure of tester entering the detector, carries out the humidity reduction and resistance/capacitance matching of the airflow, and realizes the best airflow condition of the detection. The detection module is provided with the sensitiveness and selective requirement meeting the expiration detection standard requirement, and a self-marking function on the zero point and sensitiveness calibration. The control module carries out all necessary automatic controls on the sampling and detection, carries out the information processing, memorizing and communicating, selects various function by a touch screen user interface, inputs the information of the tester, guides the detection process, records and displays the detection results.