Abstract: Herein, we addressed the stability of coaxial-waveguide gyrotron traveling-wave tube (TWT) with distributed wall losses.The effect of the lossy layer thickness on the stability of W-band TE₀₂ mode coaxial-waveguide gyro-TWT was mathematically modeled, theoretically analyzed and numerically simulated.The simulated results show that the lengths of linear/nonlinear interaction sections have a major impact.For instance, the lengths are responsible for the change of the most risky competing mode and for the shift of local axial oscillation of competing mode.So the optimized section-lengths stabilize the interaction.Moreover, when it comes to stability, the coaxial waveguide outperforms the conventional circular waveguide because it rarefies the mode spectrum and reduces the neighboring modes competition.For a 60 kV and a 13 A electron-beam with velocity-spread of 3%, the newly-designed W-band coaxial waveguide gyrotron amplifier had a peak power of 232 kW and-3 dB bandwidth of 2.5 GHz.