In this paper, the processing of micro hole on thick alumina ceramic by using a QCW high-power fiber laser was studied. The influence of laser process factors such as peak laser power, pulse width, pulse frequency, and defocusing amount on micro-hole shape and quality was primarily explored. The experimental results show that micro-holes with high roundness, minimal taper, reduced slagging, and no cracks can be achieved on thick alumina ceramic plates at a peak laser power of 5.0 to 6.5 kW, a pulse width of 4 to 8 ms, a frequency of 50 to 100 Hz, and a defocusing amount of -3 to -4 mm. The radius of the micro-hole can be controlled in the range of 130~400 μm, and the taper of the hole can be varied in the range of -1.4°~-0.6°. By modelling the temperature field of the drilling process, it is discovered that the accumulation of heat near the exit of the micro-hole is the primary cause of the machined hole shape′s inverted taper.