In this paper, a new design method for a piezoelectric shunt circuit to reduce unwanted vibration of flexible structures is proposed. Admittance is introduced to represent electromechanical characteristics of piezoelectric structures and to predict the performance of piezoelectric shunt system. It is shown that admittance of the piezoelectric structure is proportional to the dissipated energy in the shunt circuit. Admittance is used as a design index to construct the piezoelectric shunt system and obtained by finite-element method. The location, area, shape, and material property of piezoelectric patches are determined by admittance analysis. Vibration reduction of the piezoelectric structure with shunt circuit is realized by experiments. It is proved from damped system response of the piezoelectric structure in frequency and time domains that the admittance is proportional to the performance of the piezoelectric shunt system. A flow chart for design procedures using admittance analysis is presented to save design cost of the piezoelectric shunt system.