Abstract: The transient rotor angle stability has the risks of multi-group instability mode. Once this instability mode occurs, there will be an extremely high risk of power system collapse. Firstly, a three-cluster generators equivalent interconnection model is established, and the electromagnetic power output characteristics and the mechanism of evolution into three-group instability mode are revealed when the rotor angle swing of the leading and the lagging groups increases after the two-group instability model occurs. Secondly, the distribution law of the voltage amplitude and phase and the characteristics of the voltage zero point migration are analyzed, and the developing process of the multi-group instability mode is analyzed. Thirdly, based on the identification of critical branch by the branch transient transmission capability (BTTC) index and its response characteristics, the blocking control strategy of three-group, multi-group instability mode and the control strategy of recovery are put forward. In view of the multi-cluster hydropower dispatch system and its severe fault impacts in Sichuan province, the simulation results show the effectiveness of the blocking and restoring control of the multi-cluster instability mode.