The underwater target localization problem is one important application for underwater acoustic sensor networks. This paper, based on "underwater acoustic transmission theory", solves the target localization problem. The proposed algorithm firstly studies the principle of underwater acoustics and applies it to set up the model of underwater signal energy transmission. According to the model, the maximum likelihood (ML) method is used to estimate the target position based on measuring the received signal energy by sensor node. We also derivate the Cramer-Rao lower bound (CRB) based signal model in this paper. Lastly, our numerical simulation results have verified the feasibility of the proposed localization algorithm, and have studied the precision performance of the algorithm.