Abstract: The structure question about feature of harmonic clan for demodulation-spectrum was studied by using the envelope model of noise radiated from double propeller. The pseudo-propeller phenomenon discovered in a sea test was analysed, and structural aberration of harmonic clan was explained theoretically. Moreover, the fine analysis method of propeller noise feature was investigated by using the model-based technology, and the fine structural feature of envelope waveform was obtained by using the method. The analysis results of ship noise show that the fine analysis of propeller noise feature redound to obtain more physics feature and periodic modulation information of the propeller, and to improve recognition rate of the ship target.