In this paper， the effect of misaligned electron beam on the beam-wave interaction has been studied for a 0.22 THz confocal waveguide gyrotron traveling wave tube （gyro-TWT） with linear and nonlinear theory. The effect of electron beam misalignment on linear gain， critical current of absolutely instability and the exciting situation of backward wave oscillation is investigated by dispersion equation. A self-consistent nonlinear theory for confocal gyro-TWT is introduced to analyze the influence of electron beam misalignment on output power and beam-wave interaction efficiency. Meanwhile， the velocity spread is also taken into account to investigate the effect of electron beam quality on the confocal waveguide beam-wave interaction. The results show that the electron beam misalignments can cause efficiency degradation.