The target positioning precision of Traditional long baseline (LBL) positioning algorithm is greatly affected by the noisy time delay measurements and unknown signal propagation sound velocity. Aiming at this problem, this paper proposes a high-precision positioning algorithm, which builds a positioning model based on the Maximum Likelihood Estimation (MLE), using the time delay measurements as observation information. With the use of semi-definite programming (SDP), the equivalent average sound velocity estimate and the target position can be calculated. The theoretical deduction formula of the positioning model is given. And the simulation results show that the positioning error of SDP in different positioning points can be less than 0.35m, which has better positioning precision than traditional LBL positioning algorithm. Therefore, the positioning algorithm based on SDP has great positioning precision and stability.