Abstract This paper introduces an automatic conversion system for locomotive passing through the OCS phase break. This system bases on the programmable controller SIMATIC S7-200 CPU226 as system center, it collects locomotve signsls by identification sensor, it controls two high voltage vacuum switches. Then the locomotive can pass through the phase break with electricity.The system operates regularly and property is reliable. Key words power locomotive; Overhead Contact System(OCS); phase break programmable controller