Abstract: Sheet-beam traveling wave tubes (TWTs) are critical vacuum electronic devices that have garnered widespread attention as terahertz radiation sources owing to their advantages of high frequency, high power, and broad bandwidth. Owing to the significant size scaling effects in the terahertz band, realizing stable propagation of sheet beams through a narrow beam tunnel poses a prominent challenge to the design of electron optical systems. To address this challenge, this paper proposes an immersed flow sheet-beam electron-optical system for G-band TWTs. The system comprises a parallel sheet-beam electron gun and an immersed uniform magnetic focusing system. Simulation results indicate that the designed sheet-beam electron gun operates at an accelerating voltage of 23 kV with a cathode emission current of 127 mA, yielding a corresponding beam current density of 182.47 A/cm². To guarantee stable propagation of the parallel sheet beam, an immersed uniform magnetic focusing scheme is employed, with a peak magnetic field strength of approximately 1.02 T. The co-simulation results demonstrate that the sheet beam propagates 65.7 mm through a beam tunnel with a cross-section of 0.8 mm × 0.15 mm, achieving an electron transmission efficiency of 99.5%.