Abstract: Since the sea clutter spectrum of over-the-horizon radar(OTHR) always has large spreads due to the ionospheric multimode propagation, the clutter suppression needs to be done for detection of ship targets. The information of a priori mode number for the sea clutter spectrum can be acquired via ionospheric backscatter sounding. The effects of the identification for propagation mode number on the suppression of sea clutter are simulated and analyzed based on two common methods of sea clutter suppression, namely, the singular value decomposition (SVD) algorithm and the phase information of Fourier transform algorithm. The results show that target false-alarm or false-dismissal will occur if the identification of propagation mode number is not correct. Furthermore, the influence of the distance between the target and the first-order sea clutter and the Doppler resolution on the sea clutter suppression using the above two methods is studied through both simulated and experimental data with the precondition of known propagation mode number, and consequently two conclusions are obtained. The first conclusion is that the closer the target is to the first-order sea clutter, the worse the suppression of sea clutter operates, and when the distance of the two is within the Doppler resolution range, neither of the two methods works. The second one is that the sea clutter suppression effect is poorer with the lower Doppler resolution, and closer distance between target and clutter will require higher Doppler frequency resolution.