In order to comply the real-time online monitoring for back-to-back DC transmission project losses, this paper designed a loss monitoring system, which mainly containing loss monitoring device and host computer software platform. The loss monitoring device, based on two data collection schemes, can adapt to the needs of different projects and be compatible with lots of types of AC and DC signals, thus realize the time synchronization between rectifier side with inverter side and AC side and DC side, completely eliminate the problem of deviation inconsistency between multiple energy metering devices, therefor the high-precision real-time monitoring of loss of various parts of the system can be achieved. The host computer software can display, store and analysis large amounts of data, thus provides data support for malfunction warning, health status assessment and the power loss prediction of power supply department based on big data. This system is of great significance for many types of DC transmission engineering in loss monitoring.