A simple method based on the physical characteristics to get the main parameters of folded waveguide traveling wave tube (TWT) circuit is presented. Providing the operating frequency and beam voltage, the authors can obtain the initial structural parameters of the FWTWT. A D-band folded waveguide circuit was used to verify this method. The cold characteristics including dispersion relation and interaction impedance were analyzed. The simulation results show good agreement with their theory formula analysis. The folded waveguide slow-wave structure has flat dispersion relation, fairly high interaction impedance about 3.5 ohms at the center frequency of 220GHz. The large signal performance of 27mm (50 periods) folded waveguide circuit was predicted. Simulations show the nonlinear gain is 13.5dB at 220GHz where beam voltage and current are 20.6kV and 15mA, respectively. A saturated 3dB bandwidth is 11GHz (213~224GHz). The Microfabrication process to produce the folded waveguide circuits was discussed. The first example of the folded waveguide circuit was fabricated by UV-LIGA process.