A transient three-dimension model was developed to study the thermal behavior and solidification characteristics during laser linear welding of 304ss and Ni considering the free surface deformation of the melt pool. The evolution of temperature and velocity field, the weld pool configuration and the solidification microstructure were investigated numerically and experimentally. A moving mesh method based on the Arbitrary Lagrangian Eulerian (ALE) was adopted to capture the free surface. Dimensional analysis was carried out to study the different heat transfer mechanisms. The solidification parameters, including the cooling rate (GR) and the morphological parameters (G/R The molten pool configuration on both sides changed from circular to elliptical as a result of the change of the dominant heat transfer mechanism. Sharp changes of the curvatures of the weld pool boundary were also observed and explained by the Prandtl number. Because the G/R was bigger at the bottom and the GR was larger at the top surface of the molten pool, the microstructure morphology changed from equiaxed dendrites to cellular dendrites to planar, and grain size increased from the top area to the bottom in 304ss side. The similar phenomenon was also observed in Ni side.