High-frequency sky wave detection equipment must rely on the ionosphere as the propagation medium in the early warning and sea state remote sensing tasks. The ionosphere is time-varying and unstable, which will change the characteristics of the high-frequency electromagnetic wave propagating through it, resulting in the broadening of the echo spectrum, thus seriously affecting the detection of targets and the inversion of sea state parameters. The reason and mechanism of the echo spectrum expansion are analyzed in detail from the dispersion effect, phase contamination and multimode propagation. The bandwidth of the dispersion effect is different from that of the high frequency detection equipment. When the bandwidth of the sky wave equipment is 3–30 MHz, the bandwidths of the dispersion effect are 41.6–57.4 kHz and 0.17–10.8 MHz. The multi-quasi-parabolic ionospheric model is used to discuss the frequency selection measures to avoid multimode propagation. The modulation process of ionospheric contamination to echo is studied theoretically. It is shown that the non-linear phase contamination will cause the energy of echo to diffuse in frequency domain and to be unable to accumulate. To solve the problem of phase contamination which is difficult to solve in practice, a contamination correction method without estimating the instantaneous frequency of the echo is proposed. In the method the consistency principle of signal subspace and signal frequency vector expansion space is used, and therefore the phase contamination term can be well estimated. Based on the real data, the contamination correction results from the proposed method, phase gradient autofocus method, maximum entropy spectral analysis method and time-frequency processing method are given. The results show that the new method is a better method and can effectively sharpen the broadened echo spectrum.