The stochastic resonance(SR) characteristics of two bistable systems connected in series have been investigated. It is deduced that the first bistable system of the cascaded system is most important because of its converting a white noise into a color one, and the subsequent bistable systems continuously concentrate noise energy into lowfrequency area in terms of Lorentzian distribution, which weakens the highfrequency dithering of the last output waveform and outlines the basic shape of the wave. The law of the signal spectral amplitude varying with noise intensity indicates that within a limited lowfrequency band, the cascaded system is able to promote the height of the signal spectral spike by a certain amount of noise. If the former of the cascaded system does not stay in SR, the latter could not optimize the former output to become a SR state. With the increase of the number of the bistable systems in the cascaded system, the lowfrequency area where most of the noise energy is collected will be narrowed gradually, and hence the signal peak is easily reduced and submerged in noise. For this case, in spite of using the twice-sampling technique to improve the signal spectral spike, the improvement is limited. Therefore, a single bistable system is effective for signal detection.