The piezoelectric energy harvester obtains the optimal power within a narrow frequency bandwidth nearby its resonant frequency. The multiple piezoelectric energy harvesters (PEHs) can be arranged into an array for widening the frequency bandwidth. The ratio (Δfr/BW) of the frequency differences Δfr (│f1-f2│) to the bandwidth of a single PEH (BW，that is halfpower points corresponding bandwidth) has been defined to estimate whether the array could widen the bandwidth（BWa）. The series and parallel array PEHs have been studied theoretically and experimentally. The results show that the bandwidth of the single beam is 3.5 Hz, the Δfr/BW should be less than 0.657 when the three PEHs are in parallel, and the Δfr/BW should be less than 2.571 when the three PEHs are in series. Finally, the output voltages amplitude of beams and the phase difference among excitation on output power of array were studied through simulation and experiments respectively.