The stochastic resonance (SR) for a stochastic bistable system driven by a static force and a periodic square-wave signal as well as by additive white noise is considered from the view of signal-to-noise ratio (SNR). It is found that the SNR appears SR behavior when it is plotted as a function of the additive noise strength or as a function of the system parameters. Moreover, the influence of the static force is opposite to that of the amplitude of the stochastic potential.