Currently the usage of grid-connected dispersed generation systems is decreased in the islanding situation since dispersed generation systems（DGS） stop power supply when islanding occurs. This paper proposes hierarchical islanding control strategies by hierarchical islanding control units and PV grid-connected inverter for continuous running of grid-connected DGS in islanding situation， analyzes main functions of hierarchical islanding control units and controlling methods of grid-connected inverter when automatically adjusting output power and grid-reconnecting. Finally， simulation and experiment results show that the controlling strategies make the DGS easily work in both grid-connection models and islanding running models， and enhance the usage of the DGS.