The invention provides a passive target positioning method based on an underwater acoustic sensor network. The method comprises the following steps of: randomly arranging a plurality of acoustic sensor nodes to form a parallel network topology structure, namely the underwater acoustic sensor network together with a fusion center (FC); and acquiring target radiation noise data by using each acoustic sensor node, solving energy of a target, which is received by the node, transmitting the solved energy to the FC for fusion processing by using the node, and positioning the target according to a relationship between energy decrement and the distance between the node and the target. By the method, passive target positioning accuracy is improved; positioning robustness in environment with a few sensor nodes and low signal to noise ratio (SNR) is high; and a positioning device based on a received signal strength (RSS) wireless sensor network (WSN) is easy to implement.