Abstract: The 2.5 MeV wastewater treatment electron linear accelerator uses a 1.5 MW klystron as the microwave power source, which provides a microwave power no less than 1.3 MW at the entrance of the traveling wave tube, and accelerates the electron beam to 2.5 MeV/5 kW in a traveling wave tube with length of 76 cm. The operating frequency of the accelerator is 2 856 MHz in the Sband. The traveling wave tube adopts a diskloaded waveguide structure, which is simple and stable. To ensure that the beam can reach sufficient energy and power, the traveling wave tube is composed of a variable phase velocity bunching section, a constant phase velocity bunching section, and a light velocity section. The physical design of the traveling wave tube was introduced in detail. Using SUPERFISH, a twodimensional cavity model was established, effective shunt impedance, attenuation constant, cavity wall loss, and other key parameters were calculated, and the traveling wave tube field distribution was obtained based on the power loss. Using numerical calculation methods, the phase oscillation equation and the beam envelope equation were solved, the capture efficiency of longitudinal motion is about 50% and the beam energy spread is about 7.2% with optimizing the length of the constant phase velocity bunching section. The solenoid magnetic field that was calculated by the SUPERFISH software was substituted, and magnetic field distribution to make the beam envelope smaller than the beam aperture of the accelerating tube was optimized. Finally, the design was verified with PARMELA. When 10 000 particles are injected, 6 106 particles can be accelerated to the exit of the acceleration tube, that is, the capture efficiency is about 61%. Good consistency is obtained. Then, the threedimensional RF structure model was established. The accelerating cavity model in the simulation software was calculated using the principle of the probe method. The coupler model was calculated by using three frequency methods, and the coupling degree was optimized by adjusting the size of the coupling port and the inner diameter of the cavity. The field distribution of traveling wave tube was analyzed by timedomain method, and cavity inner diameter was adjusted to optimize field distribution. The simulation field distribution is consistent with the beam dynamics design. Voltage standing wave ratio of operating frequency is 1.01, and bandwidth of accelerator tube is 2 MHz. The design of 2.5 MeV traveling wave accelerator tube is completed, providing a reference for its research and development.