Abstract: The bistable harvester can achieve wide band and high-efficiency energy harvesting performance under low frequency and low excitation levels. Previous studies mainly use a simple resistor circuit to capture the energy in the bistable structures. This paper proposes a two-degree-of-freedom (DOF) nonlinear system formed by coupling a three-spring bistable structure with a nonlinear RLC (resistance-inductance-capacitance) resonant circuit for energy harvesting enhancement. The nonlinear electromagnetic coupling coefficient between the circuit and structure is obtained by the special configuration between permanents and coils. The governing equation of the two DOF nonlinear systems is acquired. The analytical responses of the current and displacement are derived by the harmonic balance method, whose stability is judged by the Jacobin matrix. The analytical solution is compared with the numerical solution. Results demonstrate that introducing a nonlinear two-order resonant circuit into the bistable energy harvester can further improve the harvesting responses and broaden the energy bandwidth. With the same circuit parameters, the nonlinear resonant circuit can achieve lower frequency energy harvesting performance through frequency doubling of the current compared with the traditional linear circuit. One can enhance the energy harvester performance by decreasing the resonant ratio between the resonant circuit and bistable structure, increasing the excitation amplitude, and decreasing the distance between two static equilibrium points. The system can realize the switching of single-period response, multi-period response, and chaotic responses by adjusting the resonant ratio between circuit and bistable structure, and excitation amplitude.