Abstract: Acquiring robust and continuous global positioning information for underwater targets in nuclear power plants remains a critical engineering challenge due to confined operational spaces, complex water refraction, and severe multi-path reflections from stainless-steel pool walls. To address these limitations, this paper proposed a novel underwater global positioning system that intelligently integrated a passive trinocular vision architecture with a high-precision two-axis tracking turntable. Rather than relying on traditional localized visual odometry, the proposed system was explicitly designed to establish a unified and highly accurate global coordinate framework for large-scale spatial tracking. Initially, a specialized underwater optomechanical device was developed, utilizing blue-light active illumination and narrow-band filtering to effectively suppress ambient optical noise. Methodologically, a comprehensive full-parameter calibration pipeline was introduced. This pipeline seamlessly encompassed the establishment of a static global coordinate system, the joint optimization of camera intrinsic and extrinsic parameters to compensate for underwater refraction, and the spatial eccentricity calibration between the camera array and the turntable’s mechanical rotational axes via algebraic spherical fitting. Furthermore, a real-time 3D reconstruction algorithm based on multi-view epipolar geometry and an inverse kinematic spatial transformation model were constructed. This algorithmic framework enables the continuous and unified output of the target’s absolute global coordinates, even during large-scale dynamic tracking. To rigorously validate the system’s overall performance, a 5-meter-deep simulated closed stainless-steel pool test platform was constructed. Extensive experiments were systematically conducted at varying radial distances of 1, 3, and 5 m. Quantitative results demonstrate exceptional accuracy: The average absolute positioning error for a single target is 0.871 mm during a 100-500 mm reciprocating motion; The maximum relative positioning error for a static rigid body is 1.245 mm within a 5 m radius. Crucially, during challenging large-range dynamic tracking at a 5 m depth, the system maintains a highly reliable average absolute error of 11.959 mm. Ultimately, the proposed system exhibits high precision, strong anti-interference capabilities, and excellent dynamic adaptability, providing a highly deployable spatial localization solution for complex underwater maintenance and inspection tasks in the nuclear power industry.