In this paper, the thermal impact on a metamaterial absorber (MMA) used in traveling-wave tube is investigated. This MMA has three layers, where the dielectric layer, SiC, is middle layer with metallic layers, copper, are top layer and bottom layer. The thermal impact on this MMA is studied with a comprehensive consideration of all thermal-sensitive parameter of MMA structure. The simulation results reveal the performance of MMA in a high temperature circumstances. When temperature rising from 300 K to 600 K, the absorbing frequency of the proposed MMA shifts from 27.60 GHz to 26.79 GHz. These results indicate the MMA has a potential application in TWT.