In order to mitigate narrow-band interference in DS system, a phase spectrum reservation processing is proposed in DS signal receiving and an iterative algorithm is employed in the method to reduce computational complexity. The suppressing narrow-band interference principle is discussed and the performance loss in AWGN channel is analyzed. A method of calculating the performance loss is presented. The numeric simulation results of receiving DS signal by phase reservation processing are presented for variety of interference conditions. These results demonstrate that the narrow band interference can be effectively suppressed and no obvious performance loss is caused after the phase spectrum reservation processing