Yunnan Power Grid and China Southern Power Grid have became an asynchronous network.There is a new change in the out-of-step oscillation characteristics of the grid.In this paper,four kinds of multiple serious faults are studied which are under the typical mode of Yunnan Power Grid.The out-of-step oscillation sections can be divided into two types,one is the parallel section and another is the long chain section.Yunnan Power Grid is currently equipped with distributed out-of-step devices,which is not ideal for the out-of-step determination and separation for these two types of out-of-step sections.A method is presented to design an out-of-step system based on the Ucosφ criterion and the voltage phase difference criterion.Three out-of-step systems are designed respectively for northwestern Yunnan,northeastern Yunnan and southwestern Yunnan.The strategies of the out-of-step systems are also developed.