Abstract: A method of automatic detection of line-spectrum in DEMON (Detection of Envelope Modulation on Noise) spectra is given. The proposed method is a double threshold detection process, it implements first detection process by line-spectrum recognition, which chooses the low detection threshold and makes use of the characteristics of line-spectrum; then delays some time and implements the second detection process, which makes use of the information about line-spectrum of past and followed multiple frames DEMON spectra to get rid of noise points and produce missed line-spectrum points. The analysis shows that this method can automatic detect line-spectrum in DEMON spectra with high detection probability and low false probability without knowing anything about amplitude, position, number, start and end time of amplitude modulated broad-band signal in advance at low signal to noise vatio.