In this paper, a novel testing method for the assembled performances of the helix slow-wave structure (SWS) with a supported rod is proposed based on heat distribution of SWS. A distributed temperature testing platform using fiber Bragg grating (FBG) and thermocouple is designed, and the influence of thermal sensors on the heat dissipation capability of the SWS is analyzed means of ANSYS. The experimental system including a distributed micro FBG sensor array is set up to test the assembled performance of SWS of the X-band TWT. The results show that the assembled performances of the SWS can be effectively obtained by the tested curve of temperature difference.