Abstract: A collaborative target localization algorithm in Underwater Acoustic Sensor Networks (UASN) is proposed. Comparing with other present algorithms about target localization, the algorithm is particularly considered time synchronization with high transmitting delay and acoustic multi-path propagation channel for UASN. By the condition of using new protocol of time synchronization, the target location is estimated by maximum-likelihood methods based on the range difference of wave arrival (RDOA), as the statistic model between the measuring and actual location is founded considering signal envelop in underwater channel. Meanwhile, the proposed algorithm adopts the distributed-centralized computation methods which degrade the node transmitting energy in underwater sensor networks. The result of simulation has analyzed the performance of localization precision.