We study the motion of a particle in a bistable potential in the presence of single frequency signal and dual-frequency signal, and give the approximate analytical relationship between response peak-to-peak value and driven peak-to-peak value of the bistable system under the control of dual-frequency signal. The paper also reveals a peculiar phenomenon of the non-linear system, that is frequency coupling of different frequencies and the energy infiltration from the frequency of input to the other. The analysis is carried out from the view point of dynamics mechanism and the frequency spectrum distribution. It deepens the understanding of the mechanism of stochastic resonance in the bistable system. The results of numerical simulation prove the validity of theoretical analysis.