The 8021 aluminum alloy is widely used in lithium battery packaging due to its excellent elongation. In this alloy, conventional casting is typically employed. However, coarse β-Al5FeSi phases inevitably form in conventional cast alloys, resulting in low elongation. Addition Cr can significantly suppress the segregation of Fe and Si and β-Al5FeSi phases, converting the original plate-like structures into fine platelets or particles. Such microstructual evolution further improves the mechanical properties of the alloy. To further improve the tensile strength and elongation of 8021 aluminum alloy, this work systematically investigates the effects of Cr addition on the morphological evolution of the second phase and the corresponding mechanical properties. The morphology, lattice structure, and phase composition of the second phase in the alloy are characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray diffraction (XRD). The experimental results show that adding an appropriate amount of Cr greatly changes the morphology of the second phase in 8021 aluminum alloy. The contents of β-Al5FeSi and θ-Al3Fe phases decrease obviously, while a new Al95Fe4Cr phase with an icosahedral quasicrystalline structure precipitates. This newly formed phase can effectively improve the plasticity and other mechanical properties of the alloy. Fracture analysis of Cr-modified samples reveals that the dimples on tensile fracture surfaces become more numerous and deeper. These features are typical of ductile fracture, which further confirms that Cr improves the toughness of the alloy by promoting plastic deformation. This study clarifies the microscopic mechanism of synergistically improving strength and plasticity via Cr-induced evolution of the second phase. It also provides a new strategy for composition design and microstructure control of high-performance aluminum alloys.