Abstract: The G-band metasurface-based traveling-wave tube (TWT) is inherently equipped with a double electron beam channel, where electron beams can be transported on both sides of the metasurface, and can be interacted with the localized enhanced electric field of the metasurface to improve the output power and efficiency of the G-band TWT. In this paper, a multi-beam electron gun suitable for the G-band metasurface-based TWT is studied, which uses eight identical circular cathodes to form a dual-row array to produce a double-layer multiple electron beam. At the same time, a three-stage focus electrode is designed to form the double-layer multiple electron beam with suitable size for the electron beam channel. The multi-beam electron gun simulation results show that when the beam voltage and the cathode radius are 24.6 kV and 0.15 mm, respectively, the total beam current is 173 mA, the corresponding emission current density is 30.6 A/cm², and the distance between the cathode emitting surface and the beam waist position is 13 mm. The static beam transportation simulation results indicate that under a uniform magnetic field of 0.2 T, the electron beams can be stably transported in the metasurface-based slow-wave structure for 70 mm. This study lays the theoretical foundation for the research of G-band metasurface-based TWTs.