It is very difficult to study the multi-swingmulti-group transient stability of power system.To the large disturbance,In the paper the model and development of Branch Transient Potential Energy Method is introduced,and its application to assessing multi-swingmulti-group transient stability of power system is studied.The simulation of the multi-machine power system show the validation of Branch Transient Potential Energy Method in multi-swingmulti-group transient stability assessment.