The energy-efficient train control is an important way to reduce the energy consumption of railways. Although the four-stage control strategy has been widely adopted in the energy-efficient for metro, the optimal driving strategy for metro when the external power flow is considered has not yet been adequately discussed. In this paper, the algorithm for the energy-efficient control of metro considering the external power flow is designed based on the energy-efficient control theory for trains in spatial-temporal area we proposed. Several case studies utilizing the Guangzhou Metro train-line data are presented to demonstrate the impact of the external power on the optimal driving strategy of metro.