The bending zone thickening of the metal sheet becomes bigger after laser bending. IPP image processing software is the measure tool of the thickness and the grain size. Then rely on temperature gradient mechanism (TGM) to research the thickening mechanism. The results show that the grain size becomes bigger in the heating process which means thermal expansion thickening (TET). In the positive bending process grains are squeezed and extended by the bending stress and forming extrusion thickening (ET). TET and ET are the principal factors of thickening appearance, and TET occupies the main position. The stainless steel layer and the carbon layer become thicker, and the thickened value of the carbon layer at least account for 69%. At last, the relation between the energy parameters (ie. power, scanning speed, scanning frequency and defocusing amount) and the thickened value is studied, which are the basis of theory and experiment for better bending mould and quality.