Abstract: A method for detecting the target echo in reverberation is proposed based on fractional Fourier transform theory when the transmitted signal is linear frequency modulation signal. The slow time-varying envelope of reverberation is computed to process reverberation to be stationary. The stationary reverberation is cut into blocks using a sliding window. A narrow-band filter is applied in FrFT domain for each block. Then the signal is recovered through inverse FrFT. Output of the system is the energy of the recovering signal. When the target echo is included in one block, the reverberation noise is reduced, and the output is the energy of the target echo. The results of numerical simulation and lake experiment show that the detection performance of the present method is better than matched filte's.