With the increasing integration of renewable and sustainable energy in microgrids, the prediction errors of these energy resources may degrade the economic efficiency of a microgrid because there is a time-scale gap between the large time-scale economic dispatch and the small time-scale frequency restoration control. In this paper, a distributed event-triggered secondary control method is proposed to deal with the economic dispatch and frequency restoration control for droop-controlled AC microgrids. The proposed control strategy can ensure economic dispatch and frequency restoration control at the same time, which reduces the operation cost of AC microgrids by bridging the time-scale gap between them. Furthermore, a simple event-triggered condition is designed to implement the proposed event-triggered secondary control, which only requires the communication between the neighboring agents when a significant change of state in the microgrid occurs, which is easy to implement and can reduce the communication burden.