With the continuous growth of global demand for renewable energy, the utilization of rainwater resources has gradually become a focal point of research. Piezoelectric energy harvesting has received significant attention due to its simple structure, high energy conversion efficiency, and self-powering capability. However, conventional piezoelectric energy harvesters are constrained by the narrow resonant frequency bandwidth and the insufficiency of waterproofing capabilities, limiting the energy conversion adaptability to variable environmental excitations. To address this issue, this study aims to design broadband piezoelectric cantilever energy harvesters for rainwater energy harvesting. Theoretical analysis, numerical simulations, and experimental validations are employed to investigate the influence mechanisms of droplet impact parameters, waterproof encapsulation techniques, and MFC cantilever structures on the electrical output performance. It reveals that the droplet's Weber number exhibits a direct proportionality with the impact force, which is distributed within the 0-80 Hz frequency range. Simulations and experimental results demonstrate that the U-shaped piezoelectric energy harvester significantly outperforms other designs in terms of broadening the resonant frequency range and extending oscillation duration, achieving an oscillation time of 23.7 s, a charge transfer of 2.82 μC, and an output power density of 37.76 W/m² under a single impact. It demonstrates its efficient energy harvesting capability over a wide resonance frequency range. Additionally, the U-shaped design also improves its waterproof performance, further enhancing its applicability in rainwater environments. This study provides a novel, universally applicable approach for rainwater energy collection, expands the application scenarios of piezoelectric energy harvesting technology, and offers theoretical references and practical guidance for the design and application of broadband energy harvesters.