To solve the low output power and narrow operating bandwidth of traditional rectangular cantilever beam piezoelectric energy harvesters, this paper proposes a novel “I-shaped” special-shaped cantilever beam structure. The fixed end width is doubled to concentrate and amplify stress, enhancing the electromechanical conversion efficiency of piezoelectric materials. The free end width is quadrupled with dual mass blocks to reduce the system’s natural frequency, achieving better matching with low-frequency ambient vibrations. Based on COMSOL Multiphysics, finite element simulations (modal, static, and harmonic response analyses) were performed to compare the I-shaped beam with the traditional rectangular beam. Results show that at the first-order resonant frequency of 28.672 Hz, the I-shaped beam achieves a maximum load voltage of 10.54 V and power of 4.38 mW, significantly higher than the traditional beam (7.4 V, 2.2 mW). Its effective operating bandwidth is widened from 1.068 Hz to 1.3 Hz (an increase of 0.232 Hz). Parametric scanning identifies 15 kΩ as the optimal load resistance. This study indicates that geometric optimization effectively improves the comprehensive performance of piezoelectric energy harvesters, offering a new design approach for low-frequency ambient vibration energy harvesting.