While many electrical train systems have the function of regenerative brake, the usage of regenerative energy is not concerned in running curve design. Therefore, peak power or waste of regenerative energy often occur in these systems. In this paper, based on the operating principle of Automatic Train Operation (ATO), by using regenerative brake as power limited brake, the energy-saving ATO running curve is designed with concern to both powering energy and maximal braking power in order to improve the regenerative energy utilization.