Abstract: In order to improve cooling capability of the collector and guarantee the multi-beam millimeter-wave traveling wave tube’s reliability and stability, thermal analysis of the collector with finite element software ANSYS is performed and simulation results are provided to obtain the main guidelines for thermal design of collector. The size, material and even structure of the collector are optimized. The results show that: (1) Thermal contact resistance is a major factor for the collector temperature rise. The temperature rise caused by thermal contact resistances is about one-third of the total; (2) Larger width of electrode contact with insulating ceramics might not correspond with stronger collector cooling capacity, there is a limit for the cooling capacity under certain work conditions;