Wire arc additive manufacturing (WAAM) offers distinct advantages, including low equipment cost, high deposition efficiency, and suitability for fabricating large-scale components. 921A steel (10CrNi3MoV) is widely used in the offshore industry and shipbuilding, making the development of WAAM technology for 921A steel crucial for manufacturing and repairing ship components. In this study, a two-dimensional melting‒solidification model was developed to investigate the melt pool dynamics and microstructure evolution during the WAAM process. This model integrates computational fluid dynamics (CFD) with the cellular automata (CA) method, utilizing the open-source ExaCA code for microstructural simulation. The model successfully predicted the temperature and flow fields, as well as the microstructural evolution within the deposition layer. Macroscopic metallography revealed columnar grains aligned with the heat flow direction, accompanied by acicular and granular ferrite, as well as bainitic morphologies. The simulated forming dimensions at various deposition speeds agreed reasonably well with the experimental results. An increased deposition speed reduced the fluid flow intensity within the molten pool. The ExaCA simulation demonstrated that columnar grains grew perpendicular to the fusion line, whereas nucleated equiaxed grains formed in the center of the molten pool under low-temperature gradients, obstructing the progression of the original columnar grains.