Abstract: Under the high penetration of new energy, the frequency stability of islanded microgrids is challenged by the uncertain output of new energy and the random fluctuation of load. The existing secondary frequency control schemes with auxiliary energy storage systems mainly adopt periodic trigger control schemes, which have the disadvantage of high network bandwidth occupancy. Therefore, an adaptive event-triggered secondary frequency control strategy considering auxiliary energy storage systems and uncertain delays is proposed. First, an adaptive participation mechanism considering the state-of-charge (SoC) of energy storage devices is designed to avoid overcharge and overdischarge without affecting the control structure. Second, in order to reduce the network bandwidth occupation, an adaptive event-triggered control (ETC) strategy is designed. Only when the deviations of the secondary frequency control system exceed the trigger threshold, the statuses and control instructions can be transmitted. Finally, the design criteria of ETC parameters are derived based on Lyapunov stability theory. Simulation results show that compared with the periodic trigger control scheme, the proposed method reduces the network bandwidth occupancy by 95.3%, and the absolute value integral of frequency deviation is reduced by 8.98%.