Abstract: The macro-micro model of molten pool solidification structures, which combined cellular automaton method and finite difference method, was developed and used to simulate the dendrite growth morphology as well as the solute concentration distribution during the solidification process of Ti-45Al alloy molten pool. And then the simulation results were validated by experiment. The effect of welding heat input, internal nucleation density and surface heat transfer coefficient on dendritic growth and morphology were discussed. The results indicated that the microstructures of molten pool were mainly composed of columnar and dendrite crystals. The higher heat input and lower surface heat transfer coefficient were beneficial to the growth of columnar crystals, while the larger the internal nucleation density, the more equiaxed crystals generated in the solidified molten pool. The simulation results of dendritic growth were in good agreement with that of the experimental results.