A folded waveguide traveling-wave tube (FW-TWT) amplifier which is driven by backward-wave has been studied in this paper. Folded waveguide backward oscillator (FW-BWO) is used as driven source and drives the traveling-wave tube amplifier. The working frequency of FW-BWO is 216GHz. The optimized results were simulated by 3D-Magic. By optimizing the length and height of the loss-material and the current of cathode, the output power of the FW-TWT is 96W. The length of the waveguide is only about one centimeter.