Abstract: A testing instrument is described for the box against pant of some turbojet engine used for checking the breakdown within the electric circuit and for adjusting the whole machine functions,while the box against pant is being examined. Some stimulating signals, which are similar to that of the engines,are sent to the box against pant by the testing instrument,then some respondent signals are produced by the box. We receive, check and analyze these respondent signals in order to judge whether the box against pant runs in right order. This paper introduces the system function,the constitution,the working principle and its working process.