Abstract: This paper proposes an optimal speed controlling method to reduce the energy consumption of connected and automated vehicles (CAVs). The proposed method considers not only the surrounding vehicles but also the signal phase and timing (SPAT) information. By determining the economic speed for each vehicle in real time based on the instantaneous optimization, energy consumption and signal waiting time of each vehicle can be reduced. To evaluate feasibility of the proposed method, three benchmark methods are introduced and used for comparison on the Vissim/Autonomie co-simulation platform. The results showed that, energy consumption of the proposed method can be reduced by 14.32%, 9.74%, and 73.72%, respectively in comparison with three benchmark methods.