To solve the problems associated with the narrow bandwidth and low power density in conventional piezoelectric energy harvesters, a piezoelectric energy harvester with dual circular cantilever beams spliced inside and outside the inverse direction is designed with the linear multimodal resonance method. Each of the two beams is covered with a piezoelectric layer and the piezoelectric layers are connected in series. A two-degree-of-freedom system lumped parameter model and an electromechanical coupling model are studied to analyze the frequency response and output performance. The performance of the piezoelectric energy harvester is assessed with different radians of the piezoelectric layers, achieving optimal output performance when the radian is 0. 5π. Setting the excitation acceleration at 0. 1 g, the resonant frequency is 82. 19 Hz, the open-circuit voltage is 49. 65 V, and the maximum output power is 3. 74 mW in the first-order mode. The resonant frequency is 119. 14 Hz, the open-circuit voltage is 44. 74 V, and the maximum output power is 3. 54 mW in the second-order mode. The measured results show that the structure can effectively broaden the bandwidth to 52 Hz with high power density. This piezoelectric energy harvesting system is simple in structure, easy to manufacture, wide in a working frequency band, and can supply power in the environment with large frequency fluctuation. Meanwhile, it has the advantages of small volume, high power density, and high-efficiency energy collection under multi-directional excitation. Therefore, this system can be applied to many fields, such as wearable devices, low-power devices, and so on.