Abstract: The ship radiated noise is provided with a stable part of line-spectrum, and the underwater target can be effectively detected by using the line-spectrum signal. The technology of Passive Synthetic Aperture Sonar (PSAS), processing for space gain exchange by time gain, which is used for synthetic processing the receiving signal of small aperture array along the linear motion, thereby the detection ability of the weak target is more excellent. An improved PSAS algorithm is proposed in this paper, compared to conventional aperture synthesis algorithm, it does not require the array element to overlap, and it processes the received signals in a segment of tine by the overall valuation array element changes on the time domain and the spatial domain, then it can directly carry out delayed compensation. Through theoretical analysis and actual simulation on ocean-trial, the validity and feasibility of the algorithm are proved. Moreover, it is suitable enough for practical engineering.