The Primary design of a Ka-band fundamental wave TE_(01) gyro-traveling wave tube(gyroTWT) amplifier was present.Employing a PIC code,gyroTWT was simulated numerically,and the details and the relations of parameters were presented.The results indicate that an over 450 kW output power and 50dB gain can be obtained from the amplifier by simulating under the condition of 100 kV voltage,20 A current,1.27 T magnetic field.The efficiency exceeds 22.5 percent and the bandwidth is about 5 percent.