High-speed passenger railway, referred to as "high-speed rail", is a very important mode of transportation in today's society, which have attracted social attention and have been widely studied by domestic and foreign railway industry researchers. However, the transportation capacity and transportation conditions of high-speed railways are closely related to locomotive types, traction power supply systems, and traffic information control. Among them, the traction power supply system is an important source of its energy, which ensures the normal operation of the locomotive, so it has very important research significance. This article first introduces the AT power supply mode used in today's high-speed railways. Then the power flow calculation method and principle are derived as the design of the software below will pave the way. At the end, a complete power flow calculation method is summarized and implemented on the Visual Studio platform using C# language. A power flow calculation software for the AT power supply system was established, inputting parameters such as voltage and current, using Newton-Raphson method to iteratively calculate, and the final result can be directly displayed in the software, or exported and saved with Excel. And after the test, the software can run well, and the calculation speed is relatively fast, the result is normal, within the error range, and basically meets the expected requirements.