Abstract: The software cold-test method simulating to obtain high frequency characteristic data including dispersion,interaction impedance and attenuation of coupled-cavity slow-wave circuit by MAFIA simulation is discussed considering effect of tube-envelop in the TWT. The precision of dispersion is increased to 0.29%. Then the simulation result is obtained more consistent with experiment data than the method that ignoring effect of tube-envelop. It is proved that computer cold-test simulation method may replace the experiment method.