Abstract: Weldability and joint softening mechanism of 2 mm thick 6061 aluminum alloy sheet were deeply analyzed and discussed by means of test and numerical simulation. Tensile test results showed that with the increase of heat input, macro forming of welded joints presented transition from incomplete penetration to excellent forming and burn through, and tensile strength first increased and then decreased. Under the optimal process of 2.6 kW laser power and 1.5 m/min welding speed, strength of welded joints reached 98% that of base metal, almost as strong as base metal, and softening degree of welded joints was the lowest. Microstructure characterization equipment (SEM, OM) was used to observe and measure position and size of pores, height of weld collapse, maximum distance of undercut, maximum distance of bite edge and length of columnar crystal trunk in the epitaxial solidification zone with collapse displacement, and causes of joint softening were analyzed and summarized in all aspects. A macro-micro coupled finite element model with cellular automata model (FEM-CAM) for cross scale microstructure evolution was established. Accuracy of FEM was verified by comparing simulation results under different welding parameters and molten pool morphology under experimental measurement. Taking simulation results of macro temperature field as heat input, driving force of dendrite growth was calculated based on solid-liquid interface solute balance method. CAM simulation results of primary dendrite length variation of columnar crystals along the thickness direction in epitaxial solidification zone were consistent with measurement results of OM characterization. Therefore, coupling model established in this paper could reveal dynamic evolution mechanism and growth law of dendrite during solidification process of molten pool.