A new laser bending process of sheet metals with pre stresses is put forward. The different loading models are analyzed, and desired pre stress distribution in heated zone of the sheet is acquired. The three dimensional thermo mechanical model is established. Using the thermal elastic plastic finite element method (FEM), the process is simulated numerically. Its forming mechanism is analyzed. The bending angles of the sheet metals can be improved significantly, and the deformation direction can also be controlled easily by changing the direction and value of the pre load reasonably. The bending angles of the sheet metals augment remarkably with the improvement of the pre load, and both are in exponential relationship.