Abstract: The value of 2219 aluminum alloy in the production of propellant tanks and the potential of the phase field method in microstructure prediction have attracted wide attention. However, the evolution behavior of the entire weld pool considering melt flow and dendrite solid movement is still unclear. In the current work, a multi-phase field-lattice Boltzmann (MPF-LBM) model was established, and an acceleration algorithm for massive dendrite growth simulation, a CPU parallel algorithm for a large computational domain and a Fortran solver with independent intellectual property rights were developed to realize the dynamic solidification simulation of the entire flowing weld pool. The results were in good agreement with the measured dendrite morphology and grain structure/size. The results show that the melt flow will flush the solute-rich layer at the front of the solid-liquid interface to form solute vortices, causing the equiaxed dendrites to translate, rotate and collide. The dendrite movement will squeeze the solute-rich layer and form a solute hysteresis transport effect behind the moving dendrite, which significantly affects the solute distribution. The melt flow and dendrite movement will cause asymmetric growth of dendrites and aggravate the anisotropy of the microstructure. The longitudinal weld is composed of short columnar dendrites and dense equiaxed dendrites. Solute segregation occurs in the liquid phase channels between the equiaxed dendrites and evolves into a network eutectic. The flow of the weld pool will form a large-gap equiaxed dendrites band on the top layer of the weld.