At present, there are many researches on the TWT (traveling wave tube) thermal properties. However, the investigations on the measurement of the temperature field inside the TWT are rarely involved. Based on RBF neural network model and ANSYS thermal simulation data of slow wave structure, a method for estimating the inner temperature of TWT is proposed in this paper, and the results with very small error are obtained by simulation data. At the end of this paper, a simplified model of slow wave structure is designed, which will be used to obtain real temperature data in the later stage to verify the reliability of the internal temperature estimation method.