Based on the nonlinear self-consistent theory of gyrotron traveling wave amplifier (gyro-TWA)，the influence of the reflection from the tapering output port on the beam-wave interaction in the gyro-TWA is studied. The simulation results show that the amplitude and phase of the reflection coefficient resulting from the variation of the tapering angle at the output port and the beam-wave interaction length have great effect on the beam-wave interaction for a Ka band TE01 gyro-TWA. Reasonably selecting the tapering angle at the output port may increase the beam-interaction frequency bandwidth with a relatively short beam-wave interaction length while keeping the saturated output power of the gyro-TWA at almost the same level.