Abstract: Magnetic field created by DC traction power supply system has become a serious man-made perturbation to the operations of geomagnetic observatories near big cities in China. Leakage current produced by DC traction power supply system is the key factor. Based on vacuum line current magnetic field model in railway transit, this paper presents a more general and simple model for quantitative estimation of the magnetic perturbation. It is effective in fast estimating magnetic perturbation value. Using this model we obtained two numerical example results: distance decay of magnetic perturbation and the amplitude variation at the end of railway line caused by motion of locomotive. In order to verify this model, field measurement was carried out in the vicinity of Jinghai,Tianjin. The ground structure in this area is approximately layered. Theoretical calculation and field measurement show a good agreement, indicating that at the distance of45 km, the perturbation caused byTianjinsubway can reach to 0.5 nT.