The risks of power supply in power systems attributed to stray currents caused by denser metro lines are increasing in modern megacities. Factors such as multi-layered soil and multiple metro operating lines (MOLs) make it more difficult to analyze dc coupling between the metro and power system. This article proposes a model for dc coupling of the metro and power system considering the two systems’ spatial information, the actual multi-layered soil, and the non-linear ionization of soil caused by stray currents. This article presents a dc coupling model suitable for studying multiple metro lines and large-scale power systems in modern megacities, which has an average relative error of 6.3% compared with experimental results. The neutral current of the transformer in a substation is up to 43 A (normal operation requires it to be less than 6 A in amplitude) under the influence of multiple MOL. The non-linear ionization of the soil near the power substations’ grounding network will decrease the soil resistance, this also results in greater dc currents intruding into power substations.