To overcome the shortcomings of the interface circuit for a piezoelectric generator in load matching and circuit losses, an improved passive Synchronous Charge Extraction(SCE) interface for the piezoelectric generator was proposed. Furthermore,by taking advantage of the passive electronic switch on maxima and the single-end flyback converter, the voltage across the piezoelectric element was reversed according to the control topology to output a constant power independent with the resistance. Simulated with the Multisim, the constant output power of the improved passive SCE circuit reaches 1.83 mW. Moreover, the electricity generating capacity measurements were conducted with the measuring system for the vibration-based piezoelectric generator. As shown in the experiments, the harvested power with the passive SCE circuit is 1.135 mW,which is 2.98 times of the 0.381 mW from the standard circuit.