Taking example for alumina ceramic, this paper briefly introduces the mechanism of CO2 laser-controlled thermal stress cutting of brittle materials. In order to study the distributions of temperature field and thermal stress field, a three-dimensional axial symmetric model of alumina ceramic is established by using APDL programming language of Ansys software. The normal stress σy of nodes which are in the laser scanning path, is in the process of "no stress-tensile stress-compressive stress-tensile stress-no stress" during cutting process, until the crack grows. The study reveals that laser power and the maximum temperature is directly proportional during the cutting process. While the laser power increases, the crack initiates earlier and the microcrack of the fracture surface becomes bigger with greater tensile stress. The results are proved by comparative experiments.