Laser thermal stress forming is a flexible forming process that forms sheet metal by means of stresses induced by external heat instead of by means of external force. Based on the analyzing of the influence of temperature on the thermal and mechanical properties of the materials, a 3D thermal-mechanical coupling model of laser thermal stress forming of thin steel sheet is set up in this paper. Based on ANSYS APDL developing platform, the stress field, temperature field and deformation field are obtained by numerical simulation of laser scanning of Q235 mild steel sheet for one time. Moreover, the forming progress and mechanism have been analyzed. The results show that the laser thermal stress forming is a quasi-steady-state process and its forming mechanism belongs to temperature gradient mechanism.