Aiming to address the bottleneck of low output power in terahertz band traveling wave tubes and responding to the distinct demand for compact design, this paper proposes a 0.34 THz folded waveguide traveling wave tube structure with power combination inside the tube. Firstly, the high-frequency characteristics of the folded waveguide slow-wave structure are investigated. Electromagnetic full-wave simulations are used to obtain its dispersion characteristics and coupling impedance. The normalized phase velocity at 0.34 THz is 0.248 and the coupling impedance is 0.46 Ω. Secondly, a 3 dB directional coupler structure for in-tube power combination is designed. The analysis indicates that its amplitude balance is within ±0.19 dB in the range of 0.31–0.368 THz, and the isolation exceeds 24 dB. Finally, the basic structure of the folded waveguide traveling wave tube based on the in-tube combination of the 3 dB directional coupler is demonstrated. The simulation model is constructed, and the results show a maximum output power of 9.16 W, a gain of 26.6 dB, and a 3 dB bandwidth of 21 GHz. For comparison, the output power of a single folded waveguide traveling wave tube is 6.18 W. The output power of the in-tube synthesized folded waveguide traveling wave tube is 1.48 times that of the single traveling wave tube. Moreover, compared with the design of a dual-tube assembly using conventional external power combination structure, the lateral size is reduced by at least 56.5%.