Through the particle-in-cell simulation and experimental investigation, the relation between the cathode ablation and the input voltage waveform of a magnetically insulated transmission line oscillator (MILO) is analyzed. And the key factor that results in the cathode ablation is found. The figures and process that the electron beam bombards back the cathode are obviously presented. By waveform adjustment of the low impedance accelerator, the reverse voltage of the input voltage to MILO is restrained, and the cathode ablation problem of the MILO driven by the low impedance accelerator is solved. Analysis results show that the electron beam which has been hauled from the cathode by the right direction voltage moves back, which the reverse voltage with adequate amplitude driven, and the beam bombards back the the cathode. The reverse current is formed and the cathode of the MILO device is ablated. Therefore, the reverse voltage immediately following the down slope of the input voltage is a key factor.