Through the simulation analysis of large-scale power system， it is found that the out-of-step generators may behave in two groups， the main out-of-step generator groups and the minor out-of-step generator groups. The main out-of-step generator groups can determine the out-of-step center interface tie-lines of large-scale power system， but the minor out-of-step generator groups cannot. This phenomenon facilitates a method of capturing the out-of-step center interface tie-lines， based on searching the lowest voltage points（center of out-of-step） during the system out-of-step state. By doing so， the main out-of-step generator groups can be identified. But the method maybe fails to split the minor out-of-step generators into the correct coherent out-of-step generator groups. Hence， in engineering practice， the generators which are likely to be in the minor ou-of-step groups should be equipped with the out-of-step separation equipment based on the off-line system analysis. Thus with the backup of the separation devices， further accident will be avoided.