Abstract: Energy function method is an effective approach for power system transient stability analysis. One important research direction is applying energy function method to transient stability analysis of hybrid AC-DC system, and the construction of energy function for AC-DC system is the key to the application of this method. From the perspectives of AC-DC system model and energy function construction method, this paper summarized the research results on the construction of energy functions for AC-DC system in recent years. In view of the deficiency that the existing energy functions cannot accurately reflect the interactive dynamics between AC system and DC system, it is necessary to construct an structure-preserving energy function for AC-DC system which considers the LCC-HVDC dynamics and the AC voltage variation of the converter bus. Furthermore, this paper explored the construction method of energy function for AC-DC system based on the variable fitting approach, and expounded the technical bottlenecks in the construction process.