As vacuum microelectronic devices for terahertz radiation, the design and development of Folded Waveguide Traveling-Wave Tube(FWG-TWT) amplifiers requires such electron guns that have concentrated beam flow, small emittance, compact structure, thermionic emission cathode. This paper discusses the theory of typical Pierce gun, and then determines the design parameters of gun structure by following Vaughan programming steps, lastly the structure is optimized by using simulation software. According to 0.22 THz FWG-TWT, beam output requirements of electron gun should be achieved as follows: beam-current is about 20 mA, beam-voltage 18.4 kV, minimum beam radius 0.01 cm, and particle emittance in the magnitude of mm?mrad.