Spirometer, mouthpiece tube and inspection method thereof. The spirometer includes at least the mouthpiece tube and an ultrasound detector configured to detect the ultrasound generated by the gas flowing through the mouthpiece tube, wherein the mouthpiece tube has a shell having an opened end, a closed or opened end, and an ultrasonic generator configured to be inserted into different portions of the shell in different situations. Therefore, by inserting the ultrasonic generator into different portions of the shell during expiration and inspiration, the gas flow during expiration and inspiration may be transformed into the ultrasonic signal and then maybe detected and analyzed.
