To monitor the loss of flexible DC transmission system of each device and overall system. An energy efficiency measurement system of multi - terminal flexible DC transmission is proposed in this paper. The system collects the data of dispatching master energy collection system and supervisory control and data acquisition (SCADA) system, and calculates in real time through loss calculation model to realize the functions of energy efficiency monitoring, energy efficiency statistics and economic operation analysis of whole system, converter stations, energy consumption equipment and cables under different operating states and time granularities in the flexible DC transmission system. The system provides reference for other energy efficiency measurement systems.