Abstract: The paper brings forward a serial network model of the mismatches in traveling wave tube (TWT) based on the physical phenomenon that microwave signal reflects between the two microwave mismatches at the output termination and attenuator of TWT. The effect of the two mismatches on the phase coincidence and amplitude coincidence of TWT is analyzed. The analysis indicates that the mismatches can lead to the possibility that the phase of phase coincidence TWT can't be compensated. It is necessary to measure the two mismatehes of TWT and combine with the production technique so as to reduce the reflections and improve the production quality of phase coincidence TWT.