Abstract: Based on time domain simulation and grid physical qualitative analysis, a large amount of simulation calculation in multiple operation modes of different years is performed on China s northwest power grid, which is a typical long distance transmission system.The out-of-step oscillation centers distribution, influencing factors and characteristics are summarized. For generators suffering oscillation energy impact from the islanding near the oscillation center and failure to keep synchronization with either oscillated generator group, measures to improve islanding operation are taken to ensure rational islanding of the major out-of-step units prior to choosing the islanding cross section on the principle of the recovery effect of the islanded sub-grid groups.