Extraction of ship-radiated DEMON(Detection of Envelope Modulation on Noise)spectrum is very important in the field of ship target recognition. The theory of ship-radiated noise modulation and de-modulation is studied，and the absolute value detection method is used to extract the DEMON spectrum. The line spectra sequence is extracted with the auto-recognizing and extracting method. Then the fundamental frequency of the line spectra， which is the shaft frequency of the propeller，is resolved with the greatest common divisor method. The processing results of sea trial data show that our method is effective.
