The detection and recognition of acoustic target is an important method to compensate for the weakness of radar in low-altitude detection. How to eliminate the noise mixed in the acoustic signal and to retain the useful information as much as possible is still a challenging problem.Inspired by the wavelet threshold, and according to the characteristics of low-altitude battlefield ambient noise, we proposed a new denoising method based on threshold Empirical Mode Decomposition (EMD). Firstly, the signal is decomposed by EMD to obtain the Intrinsic Mode Functions (IMFs). Then, threshold processing is made to the IMFs whose actual energy exceeds the estimated energy. Finally, the processed IMFs are reconstructed to obtain the denoised signal. To evaluate the performance of the proposed methoda simulation is performed using helicopter sound signal corrupted with typical low-level ambient noise under different signal-to-noise ratios.The performance is evaluated in terms of SNR, Root Mean Square Error(RMSE), and smoothness index, and a comparison is made with those of the previous denoising methods.The simulation results show that the proposed method is effective in eliminating most low-altitude ambient noise.