Abstract: Affected by phase shifts,the transverse intensity distribution(TID) of high-power blending mode CO₂ laser beams will changed regularly in different position along their propagation path.Because of different defocus value for focal laser beams,when the TID of positive defocus is more concentrated than that of negative defocus,the welding quality of positive defocus will be better than that of negative defocus.Therefore,the conventional knowledge on welding quality that negative defocus is better than that of positive defocus is broken.Moreover,the TIDs of focus in different processing position are also different when using "flying optics" to guide laser beam so that the welding qualities are different,too.It is generally considered that if one laser beam's quality is definitive,ies the welding quality should be the same under the same processing parameters.However,for one blending mode laser beam,because of the different TIDs in different processing position,the welding quality is also different.So it is not enough to use beam quality to judge the welding ability of a laser beam.The influence of fine structure of TIDs on laser beam welding quality is more important.The beams studied in this paper is of high-power blending mode CO₂ laser beams whose mode composition are about 20% of TEM00 and 80% of TEM02.In different processing position of flying optics,the TIDs of focus will present crater and mountain profiles.The two kinds of profile influence differentially the shape of laser welding seam.It is shows from our research that the welding quality of mountain profile is better than that of crater profile.