Gyrotron oscillators with CW moderate power have an important foreground in the industrial field. In this study, a self-consistent nonlinear computation was carried out for a 28GHz CW technology gyrotron. The result shows that a 28GHz technology gyrotron operating at the TE02 mode and second harmonic can obtain an output power of 50kW and efficiency of 28% for beam current 6A and beam voltage 32kV.