For DC railways, DC auto-transformer (DCAT) traction power supply system may solve the rail potential and stray current issues fundamentally. However, the load power may reach to millions of watts, and the starting current of trains may reach to thousands of amperes, which may lead to the high cost and volume of DCAT system. To evaluate whether DCAT system is feasible in the DC railways with high power and high current, the power analysis of DCAT system is proposed in this paper. The mathematical analysis and the relationships between the capacity of each DCAT and the load power are discussed in detail. Finally, the simulation model of DCAT system is built, and the theoretical analysis is verified by simulation results.