<p>Optimizing the mechanical properties and fluidity of hypoeutectic Al-Si alloys in high-pressure die casting (HPDC) is critical for manufacturing thin-walled components with large sizes. The performance and fluidity of castings over long flow distances depend on the precise control of solidification behavior during the complex HPDC process. In this study, an AlSi10MnMg alloy was fabricated using a fluidity test mold with three channels of different thicknesses to investigate the influence of varying TiB₂ content on the microstructure, mechanical properties, and fluidity of the alloy during long-distance filling in HPDC. Results indicate that the addition of 0.018wt.% TiB₂ significantly reduces externally solidified crystals (ESCs) and porosity contents, improving the filling distance from 1,700 mm to 1,833 mm. The reduction in ESCs in the castings by TiB₂ is attributed to its ability to promote the migration of ESCs from the shot sleeve toward the melt center, where temperature and flow velocity are higher. At a filling distance of 1,300 mm, the ultimate tensile strength (UTS), yield strength (YS), and elongation increase notably with addition of 0.018wt.% TiB₂. When the addition of TiB₂ increases to 0.036wt.%, the area fraction of ESCs in the channel increases compared to that with 0.018wt.%, and the filling distance slightly decreases to 1,796.9 mm. The mechanical properties of the alloy with 0.036wt.% TiB₂ are better than those of the alloy with 0.018wt.% TiB₂ over short distances, but become inferior beyond 1,000 mm. This work reveals the role of TiB₂ in regulating solidification and flow during long-range filling, offering new insights into the processability of HPDC Al-Si alloys.</p>