Charge injection is an important factor for the stable operation of electrical equipment and power systems. This paper employs first-principles calculation to analyze the charge injection barrier at the aluminum/polyethylene (PE) interface. The influence of chemical impurities, physical defects and different PE aggregation states on the injection barrier have been investigated. Computational results indicate that pure amorphous PE possesses a narrower bandgap than pure crystalline PE, enabling a smaller hole injection barrier in pure amorphous PE. When accounting for chemical impurities and localized physical defects, more electrons transfer from the PE side to the metal side at the interface, introducing a single trap state in the bandgap of PE. Consequently, electrons/holes can inject into the conduction band/valence band via this trap state, resulting in a lower charge injection barrier than that of pure PE. The minimum electron injection barrier (2.41 eV) was calculated by a crystalline PE model containing conjugated double bonds. The minimum hole injection barrier (2.19 eV) was calculated by an amorphous PE model incorporating crosslinked physical defects. This study investigates the influence of PE aggregation states on the charge injection barrier at metal/polymer interfaces, providing a guidance for research into injection mechanisms in semi-crystalline polymers.