This paper develops a new event-based optimal control (EBOC) method for cooperative adaptive cruise control (CACC) systems using parallel control and adaptive dynamic programming (ADP). First, based on the framework of optimal control and event-based control, CACC of connected vehicles is modeled as optimal control, and a triggering condition is designed to achieve the event-based CACC. Furthermore, the control stability of the closed-loop system under the developed triggering condition is shown theoretically. Then, iterative ADP and parallel control are utilized to implement the EBOC method. Finally, the correctness of the proposed EBOC method is verified by the application to a CACC system.