Abstract: The beam energy measurement system (BEMS) for the upgraded Beijing electron–positron collider BEPC-II is described. The elementary principle of Compton back-scattered process and its application on beam energy measurement is introduced. The construction of BEMS are depicted according to its technique functions, which are divided into four parts, that is laser and optics subsystem, insertion part, HPGe detector subsystem, and data acquisition subsystem. The application of BEMS on the tau mass measurement is also narrated briefly.