The Sheet Electron Beam Vacuum Electron Device is an Attractive Choice for Generating High Power Millimeter/terahertz Wave Radiation. the Sheet Electron Beam Gun is a Key Component for the Sheet Beam Vacuum Electron Device. in this Paper, a Novel Sheet Electron Beam Gun was Proposed for a Terahertz Traveling-Wave Tube. the Theories of Sheet Beam Gun are Deduced Based on the Round Beam Gun Theories. the Track of 24.5kV, 1A, 0.4mm8mm Sheet Beam is Gained through 3D Particle-in-Cell Simulation and the Theories are Verified. the Investigation Results Show that, the Design Method of the Sheet Beam Gun is Easy and Reliable.