A circuit model is established to analyze the possible cascading failure process of group prefire in 28-stage linear transformer driver (LTD). The results show that when a 4-stage group is triggered by mistake, it will cause the entire 28-stage LTD chain action, and the generated fault voltage cannot be limited by the isolation switch at the output of the 28-stage LTD. In addition, when the faulty group is located downstream of the LTD, the voltage propagating upstream along the transmission line will produce a peak field strength on the transmission line of the lower impedance of the first group that is much higher than the normal operations, which may cause the insulation failure of the device. By appropriately increasing the gap between the inner and outer conductors of the transmission line of the first group, the peak field intensity caused by the cascading failure can be weakened without affecting the output amplitude and waveform of the load current, thus improving the reliability of the operation of the device.