In order to achieve the effective detection of the line spectrum of ship-radiated noise in low signal-to-noise ratio (SNR), in this paper we improve the conventional intermittent chaotic oscillator series method, and propose a signal detection method based on the adaptive step intermittent chaotic oscillator. Through setting a sequence of the calculating steps which can cover the frequency band of the signal to be measured, the method can use just one Duffing oscillator to accomplish the searching detection for the weak signal with unknown frequency. In order to further improve the weak signal detection performance, we analyze the Holmes Duffing equation’s sensitivity for weak signal detection at different internal frequencies. Through theoretical analysis and simulation study, it is found that the Duffing oscillator has the best weak signal detection performance when its internal frequency is 0.4 rad/s. According to this result we optimize the Duffing oscillator. The simulation result shows that the signal detection performance of the improved Duffing oscillator increases 14 dB. In the end, we use the proposed method to detect a set of actual data which contain the ship radiated line spectrum, the result shows that this method could achieve effective detection of the line spectrum in low SNR.