This letter investigates the piezoelectric energy harvesting system with mechanical and electricalnonlinearities. Through impedance model, nonlinear interface circuits for energy harvestingpurpose can be simplified as an electrical impedance with real and imaginary parts. Their effectson the mechanical structure can be regarded as circuit-induced damping and stiffness. Theresulted equivalent mechanical vibratory system allows us to analytically obtain the systemresponses under different excitation and loading conditions by harmonic balance method.Furthermore, it becomes possible to achieve a comprehensive comparison among differentnonlinear interface circuits, including the standard energy harvesting circuit, synchronizedelectric charge extraction circuit, series synchronized switching on inductor circuit, parallelsynchronized switching on inductor circuit and parallel synchronized triple bias-flip circuit. Theresults from the theoretical model and experiments together reveal that the nonlinearities play asignificant role in the energy harvester’s hysteresis region size, displacement amplitude, andpiezoelectric voltage level. This investigation provides an insight into the design, optimizationand implementation for the nonlinear energy harvester.