In this article， a 220 GHz gyro-TWT operating at fundamental harmonic TE01 mode with periodic dielectric loaded （PDL） waveguide was designed through theoretical study and particle in cell （PIC） simulation. The parasitic oscillations （absolute instability oscillation and backward wave oscillation） have been successfully suppressed by study of start-up thresholds and loading dielectric rings. The beam wave interaction behaviors are compared between nonlinear theory programs and PIC simulation， the results are basically consistent. In PIC simulation， the optimized gyro-TWT was developed with saturated output power of 55.61 kW， corresponding efficiency of 26.48% at 220 GHz with 70 kV and 3A electron beam， the saturated gain is 53.56 dB， and -3dB bandwidth is 12 GHz.