Laser forming is a technique of using the energy from a laser beam to modify and adjust the curvature of sheet metals or hard materials. In order to explore the mechanism and physical process of three-dimensional (3D) laser forming of square sheet, spider scanning path was used to transform the thin square 1Cr18Ni9Ti sheet to the spherical dome by 3D laser forming. And dynamic analysis on 3D laser forming was studied. On the base of temperature gradient mechanism of two-dimensional (2D) laser forming, depending on the geometry and the thermo-physical properties of stainless steel 1Cr18Ni9Ti, the mechanism for 3D laser forming of thin square sheet to the spherical dome was developed. This paper discusses the interaction between the moving laser beam and the sheet, the temperature field on the sheet, and the step transition of strain, stress, acceleration, velocity and deformation of the nodes in laser forming. The temperature of the whole sheet is increasing during laser scanning. The heat at any time or position can cause the change of stress which shapes the circles line. But the deformation of the four corners are different from the scanning difference.