With the introduction of IEEE 1547—2003 standard， the islanding operation of distributed generation（DG） is becoming more and more concerned in the electrical industry. Considering the characteristics of islanding operation when DG runs out from system after failures， and combined with the protection and control status of distributed network which connected with DG， the theoretical analysis of power system and PSCAD simulation methods are used to study two islanding operation modes， i.e.， the short-interval power supply mode and the no-interval power supply mode. The required conditions of DG to form an island， and the impact of transient instability with different capacity of DG on the islanding operation are analyzed as well. Some related measures of control and protection are put forward for the implementation of islanding operation. A conclusion is finally obtained that the islanding operation formed by the short-interval power supply mode is currently a feasible mode.