High-proportion renewable energy has become an inevitable trend in the future development of China"s power grid. The uncertainty of power output has increased the risk of oscillation center drift, and the existing islanding control technology is facing severe challenges. In this paper, the dynamics of a multi-machine system under large disturbance can be divided into the dynamics between generator groups and the dynamics within groups, respectively. On this basis, the transient energy of the whole system can be decomposed into transient energy between and within groups. This paper pointed out that the transient energy between groups at the moment of islanding control will reduce to 0, while the transient energy within groups remains constant. Thus, the mutation of the transient energy between groups during islanding control is an important reason to improve the transient stability of the system. Based on this rule, the connection between splitting at or not at the oscillation center is analyzed from the perspective of transient energy. Finally, the above conclusions are verified by the IEEE 39 node system and an actual power grid model.