The invention discloses a device and a method for online proving of the sensitivity of a phonoscope. A standard sound source is used for issuing acoustic signals with a certain frequency and a predetermined intensity; the signals are sent to a signal detection circuit after being received by the phonoscope; the detected signal intensity is used as the characteristics of the sensitivity of the phonoscope; the variation of the signal intensity represents the variation of the sensitivity of the phonoscope; and the signals obtained by measuring the phonoscope during normal operation is compared with the signal intensity obtained by measuring the phonoscope so as to obtain the actual signal intensity without the influence of the sensitivity of the phonoscope. The invention has the advantages that the influence on the sensitivity of the phonoscope by the environmental factors of gas humidity, content, temperature, solid particle gathering, and the like is fully considered, and the accuracy in the process of measuring the phonoscope is ensured, thereby the value for accurately reflecting the intensity of the acoustic signals is obtained, and accurate, reliable and valid data are provided for the system constituted by the phonoscope.