With the realization of high speed, heavy load, and high power density in hybrid electric vehicles, the coupling vibration of the electromechanical compound transmission system has become more severe, and the transmission of vibration has become more complex. Clarifying the characteristics of vibration transmission is conducive to the pre-selection of components and parameters for subsequent vibration optimization, thereby reducing the impact of coupling vibration on the system's dynamic performance. This paper establishes an electromechanical coupling dynamic model under multi-source excitation. The vibration power is used to characterize the work done by the interface forces in the electromechanical compound transmission system, and the vibration transmission characteristics of the system are investigated. Components that generate and receive average vibration power are classified and identified, and the main components generating average vibration power are determined. The range of optimization parameters is gradually narrowed down to provide guidance for subsequent vibration optimization design and improve the efficiency of solving vibration optimization problems.