The high conductivity of metal infrastructures in tunnels induces strong electromagnetic responses, leading to signal distortion, delay, and even masking in transient electromagnetic (TEM) measurements, which reduces data reliability and interpretation accuracy. To address this challenge, a three-dimensional numerical modeling study was conducted using the finite volume method with an octree adaptive mesh to investigate the performance and applicability of the wire-source TEM method in tunnels containing conductive metallic structures. Results demonstrate that highly conductive metals generate strong secondary fields that suppress low-resistivity anomaly responses across all time stages. Compared with conventional loop sources, the wire-source system exhibits superior anti-interference capability and clearer anomaly responses under strong metallic interference. The mechanism of metal interference was revealed, and the response differences under the lateral and axial layout methods were systematically compared. An effective strategy of weakening metal interference and improving the signal-to-noise ratio by increasing the offset distance was proposed. The relative error method of the two-sided receiving points was further proposed, and the spatial positioning of the abnormal body was achieved. Meanwhile, the feasibility of the wire source in advanced detection was verified, and the effective depth of advanced detection of the wire source system in confined space was analyzed. The research results provide a new theoretical basis and methodological reference for the anti-interference detection and target positioning of transient electromagnetic methods in complex tunnel environments.