Abstract: The capability of port/starboard discrimination of dual-line array will be seriously degraded when the space between the two lines of the dual-line array is mismatched.The beam pattern of the delay-sum and delay-subtract beamforming is deduced and the port/starboard discrimination of the delay-sum and delay-subtract beamforming is also studied both under the ideal status and the status under which the space between the two lines of the dual-line array is mismatched.Analysis results show the delay-subtract beamforming has better capability of port/starboard discrimination.At last,the sea trial results demonstrate the theoretical results