Abstract: In order to detect acoustic transient signal at low SNR, a double noise reduction method of sub-wave detection and partial instantaneous energy density level is proposed. A certain order of Intrinsic Mode Function (IMF) containing useful signal is selected adaptively. Then Hilbert energy spectrum is calculated and integrated partially to filter out the residual out-off-band noise. In addition, an acoustic transient signal partial instantaneous energy density detector is established combined with width-envelope detection tactics. Lake trial data processing results show that the partial instantaneous energy density detector can reduce noise effectively and extract transient signal at low SNR. It has better performance than the traditional energy detector.