Abstract: Layer characteristics of ionosphere lead to multi-mode propagation of over-the -horizon radar(OTHR) echo signal, significant broadening of ground-sea clutter spectral, and sharp dropping of the OTHR target detection capability of ship. A selection method of single-mode operating frequency based on full-band fixed-frequency detection is proposed. The core issue of this method is to obtain the information of propagation mode number of backscatter fixed-frequency detection which changes with the distance. An extraction algorithm of propagating modes number based on feature decomposition is proposed, which achieve the extraction of model number distribution in full-band. Ultimately, the operation frequency selection of single mode is also achieved. The simulation results and experimental data verify the method is effective.