The impedance of traction network is the basis of analysis and calculation of traction power supply system. In this paper, the penetration depth of electromagnetic wave in soil is calculated for the possibility of horizontal stratification of earth in the tunnel, and the impedance expression of overhead traverse and earth loop in the case of horizontal stratification of earth are derived. Combined with the two traverse systems in the tunnel, the conductor impedance is calculated and the variation rule is analyzed. The results show that the resistance of the calculation of the impedance of the traverseground loop is greatly affected by the first layer soil resistivity. When the thickness of the first layer of soil is less than 16 m, the resistance part of the impedance of the wireground loop is greatly affected, while the reactance part is hardly affected. It can be used as a reference to calculate the impedance of overhead traverse-ground loop when the ground is stratified.