There are transition zones in the transitions between the chaotic and periodic states while blindly detecting weak BPSK (binary phase shift keying) signal by using Dufffing oscillator. The relationship expression between transition zone time length and forcing frequency of Duffing system was deduced, and the conclusion that higher forcing frequency leads to shorter transition zone time length was drawn. The simulation experiment gave the time-frequency response curve. The detection sensitivity of Duffing system will lower with the increasing of forcing frequency. The equations of output amplitude as dependent variable and the internal frequency as argument were deduced while Duffing system was in periodic state. The simulation experiment gave the amplitude-frequency response curve. Finally, by using S transform to extract the envelope of Duffing system output, the blind detection model of weak BPSK signal with the scale transformation method combined and the detection array was built. The simulation experiment results show the efficiency of the blind detection model.