In China, most DC metro systems utilize steel rails for current return. Because complete insulation between the rails and the ground cannot be achieved, part of the return current leaks and forms stray currents. When stray currents intrude into urban power grids, they can increase the transfer errors of electromagnetic current transformers (CTs) and may lead to core saturation and secondary current distortion. Existing studies generally analyze CT transfer characteristics under constant DC influence, which cannot accurately characterize the low-frequency alternating and randomly fluctuating nature of stray currents, nor their impact on CT transfer characteristics. Therefore, this paper models stray currents as the superposition of DC and multiple low-frequency components. By establishing differential equations based on the CT’s equivalent circuit model, this paper derives the mechanisms, patterns, and influencing factors of stray currents on CT transfer characteristics under both transient and steady-state conditions, which are further cross-validated through comparative simulations. The findings indicate that long-term exposure to stray currents induces DC bias in CTs, increasing the amplitude and phase angle discrepancies between the primary and secondary currents. The lower the frequency and the larger the amplitude of the stray current, the deeper the CT core saturation and the more severe the distortion of the secondary current.