<p>Selective laser melting (SLM), as an additive manufacturing technology, has garnered widespread attention for its capability to fabricate components with complex geometries and to tailor the microstructure and mechanical properties under specific conditions. However, the intrinsic influence mechanism of microstructure formation under non-equilibrium solidification conditions in SLM processes has not been clearly revealed. In the present work, the influence of Al concentration and process parameters on the microstructure forming mechanism of Al_<i>x</i>CoCrFeNi HEAs prepared by SLM is investigated by molecular dynamics simulation method. The simulation results show that the difference in Al content significantly affects the microstructure formation of HEAs, including the growth rate and morphology of columnar crystals, stress distribution at grain boundaries, and defect structure. In addition, the results show that increasing the substrate temperature improves the solidification formability, reduces microstructural defects, and helps reduce residual stress in Al_<i>x</i>CoCrFeNi HEAs. By analyzing the influence of heat and solute flow in the molten pool on the growth of columnar crystals, it is found that spatial fluctuations in Al concentration during the non-equilibrium solidification process inhibit the high cooling rates induced by steep temperature gradients. These findings promote the understanding of the forming mechanism of microstructure in HEAs prepared by SLM and provide theoretical guidance for designing high-performance SLM-fabricated HEAs.</p>