AE81 magnesium alloy castings for electric vehicle battery module ends were fabricated using high pressure die casting (HPDC). Effects of filling behavior and solidification sequence on the spatial distribution of microstructure and mechanical properties were systematically investigated. The results indicate that along the flow path toward the overflow gate, the area fraction of externally solidified crystals (ESCs) gradually decreases, and the average grain size becomes finer, resulting in a slight increase in yield strength. In addition, the pores’ volume fraction significantly affects ductility and tensile strength, with the gate region exhibiting the highest porosity (0.74%) and thus the lowest elongation (4.3%) and ultimate tensile strength (218 MPa). In other regions, the porosity decreases to 0.33%-0.39%, resulting in increased elongation (6%-7%) and higher ultimate tensile strength (235-242 MPa). Analysis of the microstructure-property relationship reveals that the yield strength follows the Hall-Petch relationship, while elongation and tensile strength are negatively correlated with pore volume fraction. This finding elucidates the mechanism behind the formation of performance gradients in HPDC magnesium alloys and provides a theoretical basis for the design of lightweight components in new energy vehicles.