A stochastic resonance mechanism of improving wideband and low frequency vibration energy harvesting of piezoelectric cantilever beam (PCB) is studied using the system nonlinearity. By adding a pair of rectangular permanent magnets, the structure of standard piezoelectric cantilever beam is improved. Results reveal that the improved PCB may be a bistable system under a nonlinear magnetic force and an appropriate distance between two magnets. Then a stochastic resonance phenomenon will happen by the input of wideband and low frequency stochastic vibrations. Under the condition of stochastic resonance, the electrical output of the improved PCB increases greatly. So the proposed method will be useful for extending the resonant band and improving the electrical output of a standard PCB.