Abstract: In long baseline underwater acoustic navigation, the position of the underwater vehicle is estimated by using the signal travel time and the equivalent sound velocity from each beacon to the underwater vehicle. However, the estimating error of the equivalent sound velocity from each beacon to the underwater vehicle will lead to large position error, and the location error increases with the increase of navigation distance. In order to solve this problem, a positioning method of underwater acoustic navigation based on particle filter is proposed. In this method, the equivalent sound velocity and the position of underwater vehicle are taken as the estimated state parameters. By measuring the travel time of beacon signal to underwater vehicle, particle filter model is established to estimate the position of underwater vehicle and to accurately estimate and track the change of equivalent sound velocity so as to improve positioning accuracy and reduce estimating error. The simulation results show that compared with conventional method, the positioning accuracy of the proposed method is improved about 4 times when the initial position of the underwater vehicle is unknown.