The variation of hole population in the resonant states of B-doped Si excited by sequences of short electric-field pulses has been investigated by the technique of time-resolved step-scan far-infrared spectroscopy. From the variation of the p3∕2 absorptions, we find that the hole population in the ground state decreases continuously with the sequential electric pulses, as a result of the breakdown delay and hole accumulation in long-lived excited states. The measured time-varying spectra of the p1∕2 series have been analyzed and attributed to a significant variation of the hole population in the resonant states. We have also observed a new absorption line at 676cm−1 which is probably caused by the electric-field induced mixing of the resonant states.