Abstract: Transporting in solenoid magnetic fields, sheet electron beams tend to manifest Diocotron instability, which could lead to collapse of beam propagation. Wiggler-field can be used to focus sheet electron beam and prevent Diocotron instability. The theory of the Wiggler-field focusing sheet electron beams was analyzed, and the corresponding beam envelope was gotten. Using 3D PIC program, the formation of Diocotron instability a Wiggler-field focusing sheet electron beam was simulated. Results show that Wiggler-field design, such as increasing the inagnitude and reducing the period properly, would restrain Diocotron instability. This might make the sheat-electron-beam travelling wave tube possible.