Resorting to particle-in-cell (PIC), simulation and experimental investigation, the phenomenon is observed that the starting voltage increases with the increase of input operation voltage of a magnetically insulated transmission linear oscillator (MILO). Therefore, PIC simulation and theoretical analysis are carried out for further study. The key factor responsible for the phenomenon is found, that is, the bigger the operation voltage and the bigger the rise slope of the operation voltage is, and the bigger the starting voltage is; when the rise slope of the operation voltage is infinite, the starting voltage is equal to the operation voltage. Therefore, the variation of the rise slope of the input voltage is the key factor that causes the variation of the starting spot for an MILO; when there is no change of the rise slope of the voltage, the starting spot for the same MILO will not change, and the starting voltage will not change either; when there is the increase of the rise slope of the input voltage, the starting voltage for the same MILO will increase accordingly. In addition, the expression of the self-insulating critical current for an MILO is modified partly.