Photovoltaic power generation is intermittent, volatile and difficult to predict, so it needs to be complemented with other energy sources. A common approach is to build water-light storage microgrid systems using the fast response of hydro units and the flexible and efficient compensation of energy storage batteries. However, such systems have strongly non-linear characteristics and a key issue for their stability is frequency control. To address this problem, this paper designs a hybrid H2/H∞ robust controller based on multi-objective H∞ theory, with the main objective of suppressing frequency oscillations in transient states. The improved seagull algorithm (ISOA) obtained by chaotic initialization and parameter improvement of the algorithm is embedded in the controller. ISOA is used to optimize the weighted matrix coefficients and norm weights of H2/H∞ robust control, and finally the optimal hybrid robust controller is obtained. The simulation results show that the designed robust controller can ensure the frequency stability of the system under power disturbance and parameter perturbation, and has satisfactory dynamic characteristics.