SUNRAY-1D large signal model developed earlier for a TWT with a cylindrical electron beam was modified for one-dimensional nonlinear analysis of beam-wave interaction of a planar THz BWO with a rectangular sheet beam. The cold RF characteristics of the SWS along with the input dc beam voltage, beam current and beam size are used to determine the RF performance of a BWO with the setting of proper boundary conditions. The RF field is set very low (0.001 μW) at the collector end and the initial complex RF voltage at each integration plane is set with the RF wave group velocity negative but the phase velocity positive at the RF frequency corresponding to the slow space-charge wave. The RF wave interacts with the forward-moving electron beam and provides beam velocity modulation and beam bunching (density modulation) which grows near the output. The induced RF field at each integration plane due to the modulated electron beam provides the backward wave growing from the output (collector end) to the gun end. A fairly good agreement of backward wave growth from the output to the input as well as BWO tuning range by varying the input dc beam voltage as simulated by SUNRAY-1D code are found with the published results simulated by the CST-3D simulation code.