A new system for the real-time loss increment measurement of reflecting mirrors is designed and built, and the loss change curve of reflecting mirrors is gained. The results show that the system is capable of effectively monitoring the loss change process of reflecting mirrors in the plasma environment and can reveal the change law. The actual loss increment is closely related to the natural standing time after the interaction of reflectors with plasma. The curve amplitude of loss change is quite different when the discharging current and the cavity pressure change. These measurement results can provide an experimental evidence for the study of the loss change mechanism of reflectors under the action of plasma.