<p>In steel-aluminum laser welded structures, there is a significant difference in material properties, which often leads to the formation of microscopic pores within the weld seam during welding. These pores adversely affect the mechanical properties of the welded joints. This study focuses on the microscopic pores in the weld seam of steel-aluminum laser lap joints, emphasizing the three-dimensional characteristics of these pores. Through steel-aluminum laser welding process experiments and X-ray computed tomography (CT) scanning, the three-dimensional characteristics of weld seam microscopic pores and their formation mechanisms were investigated. The research results indicate that the deeper the penetration depth, the fewer the number of pores, but the larger the maximum pore volume. Most of the microscopic pores inside the molten pool are small foam-like pores with high roundness. Large-volume pores are formed by the aggregation and fusion of these small foam-like pores. In most samples, the maximum pore volume is on the order of e⁻⁴mm³.</p>