High frequency system is the key part of folded-waveguide (FW) traveling-wave tube (TWT), it will directly affect the operating frequency, bandwidth, gain and other indicators of TWT. In order to obtain larger output power and higher gain than a conventional single-beam FWTWT, the basic characteristics of the 0.34 THz high order mode two-beam FWTWT are studied. Firstly, the dispersion characteristics and interaction impedance of two-beam FW are calculated and compared with the results of simulation. The results show that the theory of dispersion characteristics is consistent with the simulation results and the interaction impedance matches well in high frequency band. CST studio suite is used to simulate the beam-wave interaction of the two-beam FW, and the output power is 41.68 W. In order to obtain high output, the height of the straight is increased. And the 63.12 W output is obtained with an increase of 52.7%. High frequency system is constituted by mode converter and output window structure, and good transmission characteristics are obtained within 25 GHz bandwidth. In the operating bandwidth, |S₁₁| is greater than 15 dB, |S₂₁| is less than 4.5 dB.