We study numerically the effect of signal modulating noise in bistable stochastic dynamical systems. By making a numerical analysis of the bistable stochastic dynamical systems, we find that the effect of an input sinusoidal signal is also a sinusoidal signal in the output, and the effect of the white noise is a Wiener process. When the system parameters are small enough, the coupling effect between the sinusoidal signal and the white noise can be largely weakened. Under this condition the noise can be largely reduced in the system output, and the effect of signal modulating noise can occur in the bistable stochastic dynamical systems.