Based on the dispersion equation of electromagnetic mode and the nolinear theory of gyrotron traveling wave amplifier (gyro-TWA), the influence of the thickness of the lossy layer on beam-wave interaction in the gyro-TWA is studied. The simulation results show that large attenuation for competing mode TE21 can be realized while that of operating mode is small by appropriately choosing the thickness and conductivity of the lossy layer. The bandwidth decreases with the increase of the thickness, but the output power of the middle of the bandwidth increases and becomes more smooth. This is useful for improving the stability of gyro-TWA.