Abstract: Most algorithms for blind signal separation(BSS) have poor performance in a noisy back-ground.An instantaneous blind signal separation method based on subband decomposition is presented to deal with noisy BSS.Using a modified whitening matrix and selecting an appropriate subband,draw-backs due to background noise can be alleviated.Simulation experiments show that mixed underwater acoustic signals in noisy background can well be separated.In addition,the method is robust in the back-ground noise of different intensities.