Abstract: The structure of the cathode-heater assembly has been changed appropriately in order to reduce the heating power of some X-band space pulsed TWT. Thermal characters of cathode-heater assembly are analyzed using finite element software ANSYS, and its steady-state temperature distribution, cathode temperature transient solution and the relationship between heating power and cathode temperature are obtained. Simulation results are compared with test data, which shows that the calculation error of cathode temperature is within 1.3%, indicating the model and the methods have been correctly used. Then the structure of the cathode support cylinder is optimized by studying the relationship between heating power and cathode temperature under different slot width, thickness and material. Simulation results show that the optimized cathode-heater assemblys heating power is reduced from 8.2 W to 6.7 W.