Taking 316L/Q345 dissimilar steel laser welded joints as the research object, a bubble generation process model under keyhole instability conditions was established. It was concluded that the main reason for the formation of porosity were the retention of the bubble in the convective mixing zone and the adsorption of the solidifing interface on the bubble. The results of thermodynamic calculation showed that the change of surface free energy ΔGs of the solid-liquid interface attached by bubbles is less than 0, which further showed that the diffusion of bubbles to the solid-liquid interface was a spontaneous process; In addition, based on the fact that the characteristics of heat transfer, momentum transfer and mass transfer in the convective mixing zone are easier to meet the conditions of crystallization thermodynamics and kinetics. Meanwhile, a macrosegregation formation process was established for the cross-section of the laser welding joint penetrating through the keyhole.It is considered that the crystallization first occurs in the convective mixing zone, and the non mixed boundary layer was brought into the weld is overcooled under the action of the flow of the pool, forming “island organization”;or weld metal invaded the non-boundary mixed layer, forming a “beach organization”.