In cluster wellbores, where casings are densely distributed with complex spatial relationships, drilling new wells poses a high risk of collision. To mitigate this, high-precision ranging and positioning technologies are essential. Existing magnetic ranging techniques have proven effective in applications such as cluster wells and relief wells. However, their limitations—including susceptibility to interference from adjacent wells, high operational costs, and poor adaptability to multi-casing environments—restrict their applicability in engineering practice. In this study, a method of ranging and positioning casing based on active excitation with a high-intensity magnet is proposed. The magnet and probe are both deployed within the drilling wellbore, thus preventing any disruption to adjacent well production while also allowing operation in complex environments with multiple closely-spaced casings. By magnetizing multiple close-range casings, this method exploits magnetic field coupling effects that inherently reflect casing quantity and distribution. A combination of spatial magnetic field coupling theory and numerical modeling was employed to analyze casing magnetization under coupled field, and sensitivity studies were carried out to evaluate factors influencing coupled field strength. The optimal sensor installation position relative to the high-intensity magnet was determined, and a continuous magnetic signal-based tubing positioning model was established. The geomagnetic component synchronized with the drill string's rotation frequency is extracted from the total magnetic field and used as a reference for the geomagnetic north. After geomagnetic filtering, a relative computational model for casing distance and azimuth was developed based on simulation results. A simulation experiment on the ground was further designed to validate the reliability of the numerical modeling. Results showed that the maximum absolute error in computed casing geomagnetic azimuth was 4.22°, while the maximum absolute error in distance was 0.08 m. This study not only introduces a new method but also provides an important theoretical foundation and data support for the advancement of magnetic ranging theory in complex downhole environments, as well as for subsequent inversion studies aimed at determining spatial positions from known magnetic field distributions.