To analyze the power and spectrum performance of the Folded Waveguide(FWG) Traveling Wave Tube(TWT) sever region, a W-band TWT with four-port structure is proposed and developed. The S-parameter of interaction circuit, power and spectrum characteristic at the sever region are obtained and analyzed. The analysis shows that the power amplitude of port 2(sever of the input segment) mainly depends on the input power of the TWT at the saturated state, which is not proportional to the gain of the input segment. The power of port 3(sever of output segment) mainly depends on the matching performance of the ports, and the calculated power is in an excellent agreement with the measured data. The study in this paper provide a practical method for designing sever region of the millimeter wave and terahertz TWTs.