Abstract: For getting high processing gain,the bistatic active detection often use long-duration probing signals with the compromise of losing more weak echoes in strong blast.A method based on the Fractional Fourier Transform(FrFT)is proposed to diminish the direct blast of Frequency Modulation(FM)signals.With the sparse representation of the frequency modulation signals in the time-frequency domain,the direct blast and the echoes can be easily separated.The influence of multi-path can not be ignored because the distance from transmitter to receiver is long.Therefore,the influence of acoustic propagation in shallow water is analyzed by numerical simulation.Theoretical and simulation analysis show that the resolution of FRFT is high enough for FM signals with certain time-frequency distribution and the corresponding direct blast can be effectively separated and diminished.The proposed method is applied to the real active sonar data collected during the South China Sea experiment in April 2018.The experimental results indicate that the direct blast can be diminished excellently.Furthermore,the proposed method is stable in a shallow water environment and with low computational complexity.