Abstract-Electromechanical impedance technique is a very important research field in the structural health monitoring.This paper intends to study the mechanism of some interesting phenomena in the application of this technique, such as variation rules in electrical conductance spectra at high frequency range are different from those at low frequency range. This work will help to understand the test results of electromechanical impedance measurement better. A PZT patch attached to a multiple degree of freedom system model is established and its electrical admittance expression is also derived. By adjusting the parameters of one changing component in the multiple degree of freedom system, the rules how mass, stiffness and damping parameters influence the electrical conductance spectrum are investigated. The calculation results shows that those conductance peaks formed by PZT patch modals change in different patterns with the variation of component parameters if they are on the different side of resonant frequency of changing component. In addition, the equation of resonant frequency of PZT patch and the changing component will result in the corresponding conductance peak being divided into two.