A model toward explaining the self-initiated resistance to flashover of insulators in vacuum is proposed, thus the previously underestimated effect of self-initiated magnetic field generated by nanosecond pulsed voltage is taken into account to satisfy the experimental data of flashover voltage of insulators in vacuum, which changes with the variation of pulses' risetime. The data of several experiments is illustrated to be compared with the theoretical calculation of this model, and the differences between the theoretical estimation and the experimental results are expounded. The outcomes of this model can be used to improve the flashover voltage of Photoconductive Semiconductor Switches(PCSSs), which is highlighted in the future development of pulse power equipments.