<p>The development of new energy vehicles and their lightweight requirements promote the application of one-piece die-casting integrated structural components. The dimensions of these castings are large, and the performance of different regions varies significantly. Therefore, ensuring the mechanical properties of key stress areas is particularly important. This work focuses on the rear longitudinal beam integrated structural component made from AlSi10MnMg alloy. The microstructure and mechanical properties of the die-casting and heat-treatment states in key stress areas were analyzed, considering the filling path. The results show that both the number and distribution of eutectic silicon, intermetallic compounds, and pores change significantly with an increase in distance from the filling path. As the filling path extends from 0 to 702 mm, the eutectic silicon content increases from 45.86% to 72.66%, leading to a reduction in elongation. The quantity and size of pores in the fargate region increase compared to the near-gate region. Moreover, the micropores grow in the alloy after heat treatment. Following heat treatment, small-sized polygonal Al-Mg and Fe-Mn intermetallic compounds, measuring 2.5–7 μm, are concentrated in the far-gate region. Under the combined influence of these factors, both the tensile strength and elongation of the castings exhibit a decreasing trend as the distance from the gate increases. Research indicates that increasing the pressure injection rate and extending the holding time are primary strategies for enhancing the alloy’s toughness, provided the alloy composition and vacuum level are controlled.</p>