Abstract: The purpose of this work is to present further improved threespectrumline method to reduce frequency estimation error in interference mitigation algorithm presently used in ionosondes.New method uses the three spectrum lines near the primary interference peak in fast Fourier transform（FFT)spectrum to calculate the frequency of interference.Then，the amplitude and phase of the interference are computed with discrete Fourier transform（DFT)method at known frequency.Computer simulation of random sinusoid estimation error in white noise background reveals that the frequency error of threespectrumline method is reduced by about 50 percent versus the doublespectrumline method that has been applied to ionosondes.Observational results of applying the new method to interferences mitigation of ionosondes echoes show that the strong narrowband interferences of the echoes could be restrained by 20～60 dB and the interferences restraint KG*3capacity of threespectrumline KG*3method improved by 1～6 dB compared with doublespectrumline method.It is observed that the traces of ionograms where interferences are removed contained more sounding information and it is advantageous for inverting ionospheric characteristic parameters accurately.