Abstract: In this paper, the three-dimensional simulation code MAFIA and its quasi-periodic boundary conditions and the postprocessor are used to determine the coupled cavity slow wave structure cold-test characteristics. Some simulation methods of dispersion and on-axis interaction impedance of Hughes coupled cavity slow wave structure have been studied. Two different defintions of the whole impedance and the Pierce impedance are discussed. The electrons interact with the negative once resonance of microwave and the interaction impedance is the mean value on the beam. There is a factor difference between the whole interaction impedance and the Pierce interaction impedance. The simulation result is consistent with the actual measurement result.