Laser drilling of alumina is a noncontact material processing method, which has great advantages over the traditional mechanical machining. However, the quality of laser drilling is still a challenge. In this study, a 2D transient model is developed to simulate the underwater laser drilling of alumina, considering the recoil pressure which is generated by adjusting the density of water. The distributions of the temperature, pressure, and velocity during the drilling process are examined. The numerical results show that the underwater-drilled hole with smaller taper is obtained compared with that in air, which is attributed to the recoil pressure, higher specific heat capacity, and heat transfer coefficient of water. The experimental results validate the phenomenon in numerical simulation.