The analysis of the radiated noise of vessels given in this paper showssome strong superposed line components in low frequency speetrum below100Hz occurring at discrete frequencies which correspond with the rotationspeed of propeller shaft,or propeller blade frequency,or their harmonic fre-quencies.since the line components reflect propeller＇working characteristics,the propller＇s features can be extracted directly from low-frequency line com-ponents in addition to demodulated line component.So there are two ways toextract the features,one is direct way,the other is demodulation way.Detec-tion performance of the line component in background-noise is discussed inthis paper.The signal level is defined as the difrerence between the PDF＇s(Probability Density Function)mean of the peak of the line component andPDF＇s mean or the background-noise.In dircet way the signal level of the linecomponent is proportional to the signal noise ratio(S/N).In demodulationway the signal level of demodulated line component The analysis of the radiated noise of vessels given in this paper showssome strong superposed line components in low frequency speetrum below 100Hz occurring at discrete frequencies which correspond with the rotationspeed of propeller shaft, or propeller blade frequency, or their harmonic fre-quencies.since the line components reflect propeller’working characteristics, the propller’s features can be extracted directly from low-frequency line com-ponents in addition to demodulated line component.So there are two ways toextract the features, one is direct way, the other is demodulation way .Detec-tion performance of the line component in background-noise is discussed in this paper. Signal level is defined as the difrerence between the PDF’s (Probability Density Function) mean of the peak of the line component and PDF’s mean or the background-noise.In dircet way the signal level of the linecomponent is proportional to the signal noise ratio (S / N) .In demodulationway the signal level of demo dulated line component