Abstract: Based on the principle of coupling impedance bench measurement, the validity of this method is discussed by simulation method in this paper. Beginning with the analysis of electromagnetic field in homogeneous circular waveguide and coaxial transmission line, the physical concept and source of validity question of coaxialwire method are described and a general conclusion is obtained. Then some results obtained by other methods are reviewed briefly. Finally, the simulation method to analyze the validity problem and the main results are presented. The simulation method can not only be applied to most accelerator components, but also give more precise and direct results.