Abstract: In the ideal assumption,the optimum detection problem of Gaussian signal in the background of Gaussian noise have attracted many research works,it is the one of theoretical basis of passive sonar detection.The theoretical analysis and.experiment proved that there exist single frequency signal component in radiated noise of underwater target.In the situation of low signal to noise ratio,it is necessary to adopt new detection method,which is different with broadband energy detection.Several kinds of theoretical method for detecting single frequency component are described in this paper,including autocorrelation detection,FFT analysis,adaptive line enhancing etc.The system gain are derived.It is showed that the segment FFT detection has obvious advantage,it is more robust for the frequency shifting,which might have in signal propagation.