Abstract: The influence of large coupler on the cavity paramters of relativistic klystron amplifier is described in this paper.The parameters include the plots of electron field,resonance frequency, Q value,characteristic impetance,and coefficience of coupler.A new coupling structure is designed to resolve the influence,especially the resonance frequency,coupling coeficience and the plots of eletron field.The results is very important for the device design of high power microwave.