In this paper, a multimode steady-state theory for Gyro-TWT is established. Effects of the input power and guiding center radius on backward wave oscillation in a TE11 Gyro-TWT are studies by this theory. The result shows that either of the two effects can reduce the output power of the backward wave oscillation to a certain extent, while both of them simultaneously can more effectively improve the output characteristics and the stability of the TE11 mode.