Abstract: In the calculation of stray current separated from field and circuit, the track-to-earth resistances parameters of the resistance network model are affected by the resistivity of the ballast bed and soil in the calculation of the ground potential distribution, and it is difficult to accurately reflect the stray current diffusion distribution. This paper proposes a field circuit coupling simulation model. Through the regional equivalence of the space and conductor structure in the DC metro return system, the stray current diffusion field model is established by the direct-boundary-element method, and the stray current coefficient matrix and stray current mutual resistance matrix representing the stray current diffusion distribution are obtained and stored up. The DC traction power supply system is equivalent to the centralized circuit of multi time-varying power supply. The equivalent circuit of the traction power supply system is modified by using the stray current mutual resistance matrix, and the dynamic simulation model of stray current driven by the train diagram is established. Compared with the CDEGS software, the simulation results show that the rail potential error is within 2.04%, the tunnel surface potential error is within 1.09%, and the simulation calculation speed is increased by 83.32%. Case analysis shows that most stray current leaks from the section with large distance between traction posts.When the track-to-earth resistances of the line is greater than 3.76 Ω·km, its change has a negligible effect on the rail potential peak value. When the track-to-earth resistances is greater than 6.94 Ω·km, the rail leakage current density is less than 2.5 mA/m.