Abstract: The Duffing system will be in periodic chaos state by the influence of the frequency difference and fixed phase difference between periodic signal and forcing signal of Duffing system. The intermittent chaos state will appear due to the phase changing of BPSK signal. The weak BPSK signal blind detection model is proposed by using the sequence chart method, extracting the envelope of the sequence output with S transform, determining the working bandwidth of the detection equation, and building of the normal and reverse phase oscillator detection equations. Simulation result shows the detection of weak BPSK signal will be realized when SNR is greater than -40 dB.