Abstract: The test was carried out on the nut at the connecting part of the butterfly bolt. The precision impedance meter was adhered to the piezoelectric material on the outside of the nut to realize the function of testing the electrical admittance signal. The variation law between the peak rate of piezoelectric admittance spectrum and the bolt pre-tightening force was analyzed, and then the connection state of the bolt was monitored according to the peak impedance frequency of the piezoelectric material. The peak frequency of admittance shifted to the right under all preload conditions. According to the peak frequency, the bolt neck tightening force can be obtained, so as to infer the bolt connection situation. When the preload increases, the stiffness of the piezoelectric material will increase, and the piezoelectric material will form a larger resonance frequency after coupling with the butterfly mother.