Based on the fracture behavior during laser irradiating brittle materials,a controlling fracture technique was used for cutting brittle materials. In order to investigate the mechanism of fracture behavior during pulsed laser irradiating single silicon, a three-dimensional finite element thermoelastic calculational model which contains a pre-existing crack was established based on the heat transfer theory. The development of the temperature field and thermal stress field were investigated during the pulse duration and the changes of stress intensity factor around a crack tip were analyzed. The simulation results show that there are two tensile stress zones induced by the laser heating zone. When the laser spot is near the edge of the silicon wafer, the larger tensile stress is induced at the edge of the silicon wafer, and when the pulsed laser scans the silicon wafer, the pre-existing crack can induce the fracture to propagate along the moving direction of the laser beam. Obtained results are well coincident with the crack expanding process reported by the literature.