When grid-connected microgrid operates in island mode, the energy output of microsources and the capacity of energy storages cannot ensure the power supply for all microgrid loads; it is necessary to remove part of microgrid loads in order to maximally guarantee the power supply for the important loads and extend the power supply time of all loads as far as possible. In this paper, an optimal load shedding strategy of microgrid based on improving the reliability of load power supply is presented; according to the power prediction of microsources and loads during the operation of the island, charging and discharging power and surplus energy of energy storages are predicted, and the switching time of loads at different levels are determined by optimization. The application of this strategy improves the power supply reliability of all the loads in the microgrid while ensuring the continuous and reliable power supply of important loads. A microgrid simulation system based on the load shedding strategy is established, and the simulation results show the correctness and practicability of the strategy.