Abstract: Over-the-horizon radar(OTHR) clutter spectrum broadening caused by ionospheric multi-mode propagation considerably restricts the detection of low-speed target on sea surface. A multi-mode propagation suppression method based on working frequency selection is presented, the key point of which is to acquire the ionospheric propagation mode information of full frequency range that can be solved by backscattering detection propagation mode extracting. Combined with methods of ionospheric radio propagation theory, signal processing, image processing, statistical analysis and ray tracing, a novel algorithm to extract backscattering ionogram frontier, retral and caudal based on the quasi parabolic model is proposed, using which the regional propagation mode intelligent extraction under the multi-mode propagation is implemented and thereby the single-mode working frequency selection aiming at the specified region is finally actualized.