This paper studies the optimal configuration of Unmanned Surface Vessels (USVs) for Moving Long Baseline (MLBL) positioning system. The optimal configuration includes the optimal formation of USVs, the optimal distance between USV and Autonomous Underwater Vehicle (AUV), the optimal number of USVs, and the optimal parameter of distance measurement error. In previous paper, we establish a cost function PAM to estimate the positioning accuracy. In this paper, by minimizing this cost function, we discuss the optimal configuration of USVs. The effects of the distance between USV and AUV, the parameter of distance measurement error and the number of USVs are analysed. Simulation examples illustrate the results.