Abstract: By theoretical derivation and sample analysis,the structure law of ship radiated-noise demodulation -spectrum harmonic clan feature and correlative questions are investigated. In order to solve the problem of propelle blade-number recognition, the propelle radiated-noise model is built on the basis of five hypotheses. Further, the mathematical expressions of the structure law are educed by the model.And class space of propelle blade-number is analysed by the mathematical expressions. The analytic results can be applied in recognition as expert knowledge. Finally, the statistic experiment is done by the feature samples that are extracted by using the wavelet packet analysis. The statistical average results are consistent completely with the structure law, and the samples of 78.6% accord with the structure law basically.