Observability property has a very practical meaning in the underwater localization. This paper presents the observability of underwater localization based on the moving long baseline from the aspect of Fisher information matrix, which characterizes the amount of information provided by the measurements in the localization. The localization with two leader AUVs and a single follower AUV is analyzed. And the multiple AUVs geometry configuration which has a higher observability property is presented by maximization of the evaluation function based on the determinant of Fisher information matrix. Simulations are presented to compare the localization error of unobservable trajectory, observable non-optimal trajectory, and observable optimal trajectory, revealing the effectiveness of the analysis.