A 2D nonlinear beam-wave interaction is built to simulate the backward-wave oscillation(BWO) in a Helical traveling wave tube, based on the 1D beam-wave interaction model by taking into account the angular speed which is affected by magnetic field. The normalized oscillation start-up length with different space charge parameter is compared with that obtained from a equivalent circuit model in small signal, showing that they are close to each other. Also, the simulated results are in accordance with the measurements in a 8—18 GHz Helical TWT. The factors affecting the BWO are studied, by which some methods to suppress the BWO are improved.