The response characteristics of piezoelectric cantilever under ultra-low amplitude excitation were studied through the electromagnetic excitation. Firstly, the response of the linear piezoelectric beam was studied experimentally, and the influence of excitation amplitude were analyzed. Then, the nonlinear response characteristic of the piezoelectric beam with unilateral stopper was investigated, and the influences of excitation amplitude and gap size on the broadband response of the piezoelectric beam with the unilateral stopper were analyzed. The experimental results showed that the amplitude of the piezoelectric beam was less than 140 μm under electromagnetic excitation of 0.003 N. The piezoelectric beam with a unilateral stopper showed a piecewise linear response and was sensitive to the micron scale change of the gap size. As the gap decreased from 100 μm to 20 μm, the maximum output voltage decreased from 3.14 V to 1.17 V, and the half-power bandwidth increased from 5.8 Hz to 18.2 Hz.