The working temperature of the cathode, as the electron source of traveling wave tubes (TWTs), directly impacts the performance, stability and lifespan of TWTs. Since the cathode’s temperature cannot be directly measured during TWT operation, it is primarily determined through component temperature measurement and electron gun thermal simulation. Thermal radiation is a significant heat transfer mode in the electron gun and a major factor in heat loss. Therefore, it cannot be ignored in the thermal analysis of the electron gun. The heat loss was quantitatively analyzed, the thermal simulation of cathode-thermal shielding assembly was conducted considering contact thermal resistance and heat loss, establishing a comprehensive thermal radiation boundary. The temperature measurement test data curve of the cathode-thermal shielding assembly was fitted by adjusting the surface emissivity of the parts to obtain its value in high-temperature regions of TWT electron guns. The impact of surface emissivity and heat loss on cathode temperature was analyzed, and the accuracy of obtained emissivity value was verified through a heat equilibrium test of electron gun, and then a more precise distribution cloud map of the electron gun was obtained. The research results show that the surface emissivity of cathode is 0.65 when the temperature is between 950 and 1100 ℃. Besides, the higher the surface emissivity of electron gun components, the lower the cathode temperature, and the surface emissivity of the cathode cylinder has the greatest impact on the cathode temperature; the higher the temperature, the greater the heat loss of the heater. Without considering heat loss, the simulated cathode surface temperature is 14.4-17.5 ℃ higher. The surface temperature of the cathode obtained by component temperature measurement is 42-62 ℃ higher than that of the entire tube.