To better understand the physical processes of multi-pulse laser drilling, this study investigates the keyhole evolution and its driving mechanism in a time-resolved observation system. The evolution characteristics suggested a two-phase process of rapid penetration followed by moderate penetration. As revealed in the ejection and vaporization behavior, the keyhole evolution was dominated by ejection and vaporization during the rapid and moderate penetration stages, respectively. In a single laser-pulsed drilling experiment, the driving mechanism itself was found to be affected by the dimensionless laser power density. The effect of dimensionless laser power density on depth increment was then discussed by comparing the experimental observations with numerical simulation results. The results further confirmed the driving mechanism of the keyhole evolution. The results in this paper are useful for understanding the driving mechanism of the keyhole evolution during multi-pulse laser drilling.