Abstract: The theoretical analysis and experiment of the masker & maskee's spatial separation in azimuth showed that when masker and maskee were spatially separated, thresholds tended to drop and produce the phenomenon of spatial unmasking. The results also showed that the spatial unmasking was frequency related. The higher the frequency was, the stronger the spatial unmasking was. The maximum unmasking was 15 dB. The main cue is that when a masker and a maskee sources are spatially separated in azimuth, the monaural Signal-to-Mask Ratio (SMR) will increase because of influence of Head Related Transfer Function (HRTF). At the same time, the binaural processing of audio information by higher aural system is another reason of spatial unmasking.