Abstract: The coaxial wire method is the standard method to measure longitudinal and transverse coupling impedance adopted in current accelerator laboratories, and the question about its validity is the focus by accelerator physicists. Differing from other references, in the paper the measurement process is numerically simulated and the effect of inner conductor design parameters including the radius of inners and distance between inners on measurement results is analysed. The simulation results show that the inner conductor design will effect the accuracy of measurement result of impedance, but the spectrum properties of impedance is reasonable. 