Abstract: The low frequency line elements that the underwater target radiates usually have both high spectrum level and sustained stability.This feature could be used for increasing the detecting performance of conventional wideband energy integration method.The required spectrum level is theoretically discussed when the known line detection is better than wideband energy integration method.The relationship between line spectrum level and signal to noise ratio (SNR) when the target could be detected in line sub band is also discussed.Then the article proposes a detecting method for line spectrum target by constructing a matrix by using output DC jump to fluctuations ratio of sub band spatial spectrum and beam space.The matrix acts as a filter letting the certain frequency and azimuth of goal line target pass at most.Thus by fusing with other sub band results,the detection performance of conventional method increases.At last,the paper analyzes the applicable condition and detection performance.The simulation and trial data processing results have proven this algorithm could effectively extracts weak goal line spectrum under the condition of multi-interference.It needs no multi frame statistics.The detection effect is closer to the known line spectrum method.