To improve the model accuracy of piezoelectric generator, this article built a more accurate theoretical model for piezoelectric generator system under considering of rectifier diodes′ voltage drop based on the standard circuit. An analytical expression of output power was derived and the derivation of the equivalent quality′s displacement was presented in detail. A comparison between traditional model which have neglected the effect of rectifier diodes and the new improved model had been made and the results could be generalized as follows: the optimal resistance has a slightly trend to be larger in new improved model; the peak output power descends as the rectifier diode′s voltage drop increasing and the adaptive frequency bandwidth in new model reduces accordingly.