Effect of quasi-vacuum environment during pulsed laser ablation with high repetition rate is investigated . The comparison of ablation rates for pulsed laser drilling in steel with different repetition rates and different ambient pressures shows that the quasi-vacuum environment can be the main reason for the increase of ablation rate during high-repetition-rate pulsed laser ablation. The density distributions of ambient air are investigated in two different time scales. The experimental results show that the quasi-vacuum environment occurs in the pulse laser ablation and continues for several hundred microseconds after laser pulse. If the subsequent laser pulse interacts with the material during this time, the high ablation rate will be achieved as that obtained in vacuum.