Isolated microgrids are susceptible to multiple disturbances while they are operating independently. Fluctuations in renewable energy outputs and load variations can distrupt power balance, causing voltage and frequency deviations. And long-distance communication and network congestion may induce control command transmission delays that compromise the real-time performance of secondary control. To address these challenges, a fixed-time distributed secondary control algorithm based on a dynamic event-triggered mechanism（DETC-P） is proposed. To counter periodic disturbances, an H_∞ robust control strategy is adopted to strengthen disturbance rejection capability.For communication delay issues, model order reduction techniques are employed to simplify the dynamic models of distributed generation units, enhancing delay compensation efficiency. The proposed dynamic event-triggering mechanism adaptively adjusts triggering thresholds based on voltage and frequency deviations, significantly reducing communication burden between nodes while ensuring a lower bound on event-triggering intervals and avoiding Zeno behavior. Finally, a system model of the islanded microgrid is constructed, and simulation experiments are conducted to validate the feasibility of the proposed algorithm.