The basic characteristics of the G-band two-beam Folded Waveguide(FWG) TE20 mode are investigated. Firstly, the high frequency characteristics of two-beam FWG TE20 mode are calculated. The dispersion characteristic is calculated by the equivalent circuit method. The coupling impedance is calculated according to the definition. The results are compared with the HFSS simulation results. The results show that the dispersion characteristics differ more greatly as the frequency increases, and the coupling impedance decreases as the frequency increases. Then, the Computer Simulation Technology(CST) particle simulation software is utilized to simulate the beam-wave interaction of the FWG, the electron trajectory and the input and output signal spectrum in the slow wave structure are obtained. The results show that at the operating frequency of 205 GHz, the gain of four-part FWG is 34.74 dB.