In order to suppress the environmental noise interference of underwater targets, a line spectrum extraction method is proposed based on the polarization characteristic of underwater acoustic signals received by a single vector sensor. In this paper, the polarization filtering function of a fourth-order cumulant diagonal slice is developed, and the fourth-order cumulant diagonal slice characteristic spectrum corresponding to the maximum singular value decomposition is used to extract the line spectrum. The simulation and experimental results show that the proposed method can extract the line spectrum signal adaptively, which has a good abatement effect on environmental noise interference.