Laser-assisted pre-stress forming is a technology for the manufacturing an integrally stiffened panel. To investigate the thermo-mechanical mechanism for such forming method, experiment on laser-assisted four-point bending an AA6061-T6 aluminium alloy sheet is carried out. A three-dimensional (3-D) finite element method (FEM) simulation is conducted and verified with the experiment in aspects of both temperature histories and force-displacement curve. During forming process, the sample is bent via four point bending elastically and then heated with a continuous wave (CW) Nd: YAG laser system in a controllable manner. Since material yield strength decreases with an increasing temperature locally, pre-loaded elastic strain is partially converted into plastic strain, and elastic strain energy is dissipated into plastic work. In this paper, the forming mechanism is studied by analysing the coupled thermo-structural field. According to the final shape by simulation, the spring-back ratio is 78.0% (76.0% by experiment), and the energy dissipation rate is 33.5%.