A macro-micro coupled model is developed to simulate the competitive dendrite growths in different areas of the welding pool in the solidification process. The transient solidification conditions in welding pool are obtained by the three-dimensional (3D) macro-scale FEM model. The thermal conditions used in the micro-scale cellular automata model is obtained from the macro-scale FEM model by using the interpolation algorithm. The simulation results indicate that the micro-scale cellular automata model developed in this paper can simulate the morphologies of dendrites with various growth directions accurately. The solidification conditions in welding pool have obvious effects on the competitive dendrite growth. The dendrites with their preferential orientations parallel to the direction of the highest temperature gradient are more competitive. The morphology of grain structure is determined by the competition among different dendritic arrays. The dendritic arrays with more favorable growth direction can gradually crowd out other dendritic arrays and occupy more space through dendrite branching. The area near the central line of welding pool has a lower temperature gradient, a higher solidification rate, and a higher cooling rate in the solidification process, and such solidification conditions lead to the finer microstructure. The simulation results of the secondary dendrite arm spacing are in agreement with the experimental results under the corresponding solidification conditions.