To limit system bus short-circuit current, a method of 500 kV line out-series by adjusting the system operation mode is introduced. And the impact of protection and safety operation control device is analyzed after the change of operation mode. This paper points out that under the condition of line three phase trip, there are risks of safety control unit refusing action, caused by the logical criterion unsatisfied. An implementation method regarding two adjacent lines as a whole, by adding three phase output reinitiated relay in breaker protection, and taking their recombinant contacts as the adjacent line protection distant-trip input, to meet the logical criterion of safety system control device, is proposed. Field test results show that this method is simple and workable, and its validity and accuracy has been proved.