Abstract: Based on the bifurcation principle of Duffing oscillators excited by single frequency signals,a new method for detecting Doppler shift of faint sound signals in a strong noise back-ground is introduced.When the amplitude of input signals in Duffing oscillators exceeds a certain critical value,the chaotic state of the system becomes periodical.According to this principle,the parameter threshold of the bifurcation without noise in Duffing oscillators is determined,and the parameter is then adjusted according to the noise intensity to make the periodical output steady.When there is faint Doppler echo in noise,the system output is intermittent chaotic,and the Doppler shift can be obtained accurately by calculating the interval of chaotic state.