A set of selfconsistent nonlinear equations in the electron guiding center coordinate is derived for the study of the behavior of the coaxial Gyrotravellingwavetube(GyroTWT) amplifier. Calculations show that: the input signal grows faster and gets saturated earlier at a slightly lower radiation power with the increasing of the ratio of the inner to outer radius; power loss on the walls of the inner and outer conductor decrease with the increasing of the ratio. Using an electron beam of 90kV, 10A and velocity ratio of 1.0 with 3% axial velocity spread, 230kW peak output power is obtained by calculation at the frequency range of 34.26 to 36.78GHz, the gain, efficiency and bandwidth are 46.6dB, 25.5% and 7.0%, respectively. The maximum loss on the walls of the coaxial waveguide are